diff --git a/src/Bifrost.Web/Features/Heimdall/HeimdallAuthService.cs b/src/Bifrost.Web/Features/Heimdall/HeimdallAuthService.cs
index 0d39c2f..8d10b53 100644
--- a/src/Bifrost.Web/Features/Heimdall/HeimdallAuthService.cs
+++ b/src/Bifrost.Web/Features/Heimdall/HeimdallAuthService.cs
@@ -319,7 +319,8 @@ public sealed class HeimdallAuthService(
                 UserAccountId = userAccount.Id,
                 CreatedAtUtc = now
             };
-            if (grantMemberAccess)
+            // A suspension is an admin's decision: the claim neither lifts it nor, absent, imposes it.
+            if (grantMemberAccess && userAccount.Membership.Status != MembershipStatus.Suspended)
             {
                 userAccount.Membership.Status = MembershipStatus.Active;
                 userAccount.Membership.ApprovedAtUtc ??= now;
@@ -332,7 +333,7 @@ public sealed class HeimdallAuthService(
             userAccount,
             displayName,
             cancellationToken);
-        if (grantMemberAccess)
+        if (grantMemberAccess && userAccount.Membership!.Status == MembershipStatus.Active)
         {
             ApplicationBootstrapper.EnsureRole(
                 userAccount.Membership!,
diff --git a/src/Bifrost.Web/Features/Membership/MembershipSuspensionService.cs b/src/Bifrost.Web/Features/Membership/MembershipSuspensionService.cs
new file mode 100644
index 0000000..e4bd388
--- /dev/null
+++ b/src/Bifrost.Web/Features/Membership/MembershipSuspensionService.cs
@@ -0,0 +1,78 @@
+using Bifrost.Web.Data;
+using Bifrost.Web.Domain;
+using Bifrost.Web.Features.Shared;
+using Microsoft.EntityFrameworkCore;
+
+namespace Bifrost.Web.Features.Membership;
+
+public enum SuspensionOutcome
+{
+    Done,
+    NotAdmin,
+    NoReason,
+    NoSuchAccount,
+    NotApplicable,
+    ProtectedAccount
+}
+
+/// <summary>
+/// The one place a membership becomes Suspended or stops being. Only a platform admin may call it, every use leaves an audit
+/// receipt naming the admin, the account and the reason, and no sign-in path writes or lifts a suspension (Heimdall and GitHub
+/// sign-in leave a Suspended membership as it is). Unsuspending restores Active for an account that was once approved,
+/// Authenticated for one that never was; a pending request is not remembered.
+/// </summary>
+public sealed class MembershipSuspensionService(BifrostDbContext dbContext, AuditTrailService auditTrailService, TimeProvider timeProvider)
+{
+    public const int MaxReasonLength = 500;
+
+    public Task<SuspensionOutcome> SuspendAsync(CurrentBifrostActor actor, Guid userAccountId, string? reason, CancellationToken cancellationToken) =>
+        ChangeAsync(actor, userAccountId, reason, suspend: true, cancellationToken);
+
+    public Task<SuspensionOutcome> UnsuspendAsync(CurrentBifrostActor actor, Guid userAccountId, string? reason, CancellationToken cancellationToken) =>
+        ChangeAsync(actor, userAccountId, reason, suspend: false, cancellationToken);
+
+    private async Task<SuspensionOutcome> ChangeAsync(CurrentBifrostActor actor, Guid userAccountId, string? reason, bool suspend, CancellationToken cancellationToken)
+    {
+        if (!actor.CanManageMembership || actor.UserAccount is null)
+        {
+            return SuspensionOutcome.NotAdmin;
+        }
+
+        if (string.IsNullOrWhiteSpace(reason))
+        {
+            return SuspensionOutcome.NoReason;
+        }
+
+        var user = await dbContext.UserAccounts.Include(x => x.Membership).SingleOrDefaultAsync(x => x.Id == userAccountId, cancellationToken);
+        if (user?.Membership is not { } membership)
+        {
+            return SuspensionOutcome.NoSuchAccount;
+        }
+
+        // An admin cannot suspend themselves or another platform admin here: bootstrap admins are re-promoted at sign-in by configuration.
+        if (suspend && (user.Id == actor.UserAccount.Id || membership.IsPlatformAdmin))
+        {
+            return SuspensionOutcome.ProtectedAccount;
+        }
+
+        if (suspend == (membership.Status == MembershipStatus.Suspended))
+        {
+            return SuspensionOutcome.NotApplicable;
+        }
+
+        var before = membership.Status;
+        membership.Status = suspend
+            ? MembershipStatus.Suspended
+            : membership.ApprovedAtUtc is null ? MembershipStatus.Authenticated : MembershipStatus.Active;
+        var clipped = reason.Trim().Length > MaxReasonLength ? reason.Trim()[..MaxReasonLength] : reason.Trim();
+        await dbContext.SaveChangesAsync(cancellationToken);
+        await auditTrailService.RecordAsync(
+            actor.UserAccount.Id,
+            nameof(Membership),
+            membership.Id,
+            suspend ? "membership.suspended" : "membership.unsuspended",
+            $"{(suspend ? "Suspended" : "Unsuspended")} {user.DisplayName} ({before} -> {membership.Status}): {clipped}",
+            cancellationToken);
+        return SuspensionOutcome.Done;
+    }
+}
diff --git a/src/Bifrost.Web/Pages/App/Members/Index.cshtml b/src/Bifrost.Web/Pages/App/Members/Index.cshtml
index 0d9d546..b4d67e5 100644
--- a/src/Bifrost.Web/Pages/App/Members/Index.cshtml
+++ b/src/Bifrost.Web/Pages/App/Members/Index.cshtml
@@ -176,7 +176,7 @@
                                 </div>
                             </td>
                             <td>
-                                @if (Model.Actor.CanManageMembership && user.Status != MembershipStatus.Active)
+                                @if (Model.Actor.CanManageMembership && user.Status != MembershipStatus.Active && user.Status != MembershipStatus.Suspended)
                                 {
                                     <form method="post" asp-page-handler="Approve">
                                         <input type="hidden" name="userAccountId" value="@user.UserAccountId" />
@@ -184,6 +184,15 @@
                                     </form>
                                 }
 
+                                @if (Model.Actor.CanManageMembership)
+                                {
+                                    <form method="post" asp-page-handler="@(user.Status == MembershipStatus.Suspended ? "Unsuspend" : "Suspend")" style="margin-top: 0.8rem;">
+                                        <input type="hidden" name="userAccountId" value="@user.UserAccountId" />
+                                        <input type="text" name="reason" maxlength="500" required placeholder="Reason" />
+                                        <button class="button warm" type="submit">@(user.Status == MembershipStatus.Suspended ? "Unsuspend" : "Suspend")</button>
+                                    </form>
+                                }
+
                                 @if (Model.Actor.CanManageMembership)
                                 {
                                     <div class="stack-form" style="margin-top: 0.8rem;">
diff --git a/src/Bifrost.Web/Pages/App/Members/Index.cshtml.cs b/src/Bifrost.Web/Pages/App/Members/Index.cshtml.cs
index 2f385b7..e9b536b 100644
--- a/src/Bifrost.Web/Pages/App/Members/Index.cshtml.cs
+++ b/src/Bifrost.Web/Pages/App/Members/Index.cshtml.cs
@@ -1,6 +1,7 @@
 using System.ComponentModel.DataAnnotations;
 using Bifrost.Web.Data;
 using Bifrost.Web.Domain;
+using Bifrost.Web.Features.Membership;
 using Bifrost.Web.Features.Shared;
 using Microsoft.AspNetCore.Mvc;
 using Microsoft.AspNetCore.Mvc.RazorPages;
@@ -12,6 +13,7 @@ public sealed class IndexModel(
     BifrostDbContext dbContext,
     ICurrentBifrostActorAccessor actorAccessor,
     AuditTrailService auditTrailService,
+    MembershipSuspensionService suspension,
     TimeProvider timeProvider) : PageModel
 {
     [BindProperty]
@@ -102,7 +104,12 @@ public sealed class IndexModel(
                 .ThenInclude(x => x.RoleAssignments)
             .SingleAsync(x => x.Id == userAccountId, cancellationToken);
 
-        user.Membership!.Status = MembershipStatus.Active;
+        if (user.Membership!.Status == MembershipStatus.Suspended)
+        {
+            return RedirectToPage(); // only Unsuspend lifts a suspension
+        }
+
+        user.Membership.Status = MembershipStatus.Active;
         user.Membership.ApprovedAtUtc = timeProvider.GetUtcNow();
         ApplicationBootstrapper.EnsureRole(
             user.Membership,
@@ -124,6 +131,21 @@ public sealed class IndexModel(
         return RedirectToPage();
     }
 
+    public async Task<IActionResult> OnPostSuspendAsync(Guid userAccountId, string? reason, CancellationToken cancellationToken) =>
+        await ChangeSuspensionAsync(userAccountId, reason, suspend: true, cancellationToken);
+
+    public async Task<IActionResult> OnPostUnsuspendAsync(Guid userAccountId, string? reason, CancellationToken cancellationToken) =>
+        await ChangeSuspensionAsync(userAccountId, reason, suspend: false, cancellationToken);
+
+    private async Task<IActionResult> ChangeSuspensionAsync(Guid userAccountId, string? reason, bool suspend, CancellationToken cancellationToken)
+    {
+        Actor = await actorAccessor.GetAsync(cancellationToken);
+        var outcome = suspend
+            ? await suspension.SuspendAsync(Actor, userAccountId, reason, cancellationToken)
+            : await suspension.UnsuspendAsync(Actor, userAccountId, reason, cancellationToken);
+        return outcome == SuspensionOutcome.NotAdmin ? Forbid() : RedirectToPage();
+    }
+
     public async Task<IActionResult> OnPostToggleRoleAsync(Guid userAccountId, MemberRole role, CancellationToken cancellationToken)
     {
         Actor = await actorAccessor.GetAsync(cancellationToken);
diff --git a/src/Bifrost.Web/Program.cs b/src/Bifrost.Web/Program.cs
index 2a5ff5f..e19b8d0 100644
--- a/src/Bifrost.Web/Program.cs
+++ b/src/Bifrost.Web/Program.cs
@@ -117,6 +117,7 @@ builder.Services.AddHttpContextAccessor();
 builder.Services.AddSingleton(TimeProvider.System);
 builder.Services.AddScoped<ICurrentBifrostActorAccessor, CurrentBifrostActorAccessor>();
 builder.Services.AddScoped<AuditTrailService>();
+builder.Services.AddScoped<Bifrost.Web.Features.Membership.MembershipSuspensionService>();
 builder.Services.AddScoped<DashboardSnapshotService>();
 builder.Services.AddScoped<BridgeActionService>();
 builder.Services.AddScoped<AgentDispatchRunService>();
diff --git a/tests/Bifrost.Web.Tests/HeimdallCompletionTests.cs b/tests/Bifrost.Web.Tests/HeimdallCompletionTests.cs
index 532f669..15866d1 100644
--- a/tests/Bifrost.Web.Tests/HeimdallCompletionTests.cs
+++ b/tests/Bifrost.Web.Tests/HeimdallCompletionTests.cs
@@ -11,6 +11,7 @@ using Microsoft.AspNetCore.Authentication.OAuth;
 using Microsoft.AspNetCore.Hosting;
 using Microsoft.AspNetCore.Mvc.Testing;
 using Microsoft.AspNetCore.WebUtilities;
+using Microsoft.EntityFrameworkCore;
 using Microsoft.Extensions.DependencyInjection;
 using Microsoft.Extensions.Logging;
 using Microsoft.Extensions.Options;
@@ -864,6 +865,28 @@ public sealed class HeimdallCompletionTests
         }
     }
 
+    [Theory]
+    [InlineData(true)]
+    [InlineData(false)]
+    public async Task A_suspended_account_stays_suspended_through_a_sign_in_with_or_without_the_member_claim(bool asksForMemberAccess)
+    {
+        using var host = new HeimdallHost();
+        var accountId = HeimdallShapes.NewAccountId();
+        Assert.True((await Genuine(host, await host.StartAsync(member: true), accountId, ["member_access"])).SignedIn);
+        await host.Db(async db =>
+        {
+            var account = await db.UserAccounts.Include(x => x.Membership).SingleAsync(x => x.HeimdallAccountId == accountId);
+            account.Membership!.Status = MembershipStatus.Suspended;
+            await db.SaveChangesAsync();
+            return 0;
+        });
+
+        var again = await Genuine(host, await host.StartAsync(member: asksForMemberAccess), accountId, ["member_access"]);
+
+        Assert.True(again.SignedIn);
+        Assert.Equal(MembershipStatus.Suspended, (await host.Account(accountId))!.Membership!.Status);
+    }
+
     [Fact]
     public async Task A_token_capability_grants_no_role_to_a_sign_in_that_did_not_ask_for_member_access()
     {
diff --git a/tests/Bifrost.Web.Tests/MembershipSuspensionTests.cs b/tests/Bifrost.Web.Tests/MembershipSuspensionTests.cs
new file mode 100644
index 0000000..167d7ec
--- /dev/null
+++ b/tests/Bifrost.Web.Tests/MembershipSuspensionTests.cs
@@ -0,0 +1,152 @@
+using Bifrost.Web.Data;
+using Bifrost.Web.Domain;
+using Bifrost.Web.Features.Membership;
+using Bifrost.Web.Features.Shared;
+using Bifrost.Web.Pages.App.Members;
+using Bifrost.Web.Tests.Support;
+using Microsoft.AspNetCore.Mvc;
+using Microsoft.EntityFrameworkCore;
+using Microsoft.Extensions.DependencyInjection;
+
+namespace Bifrost.Web.Tests;
+
+/// <summary>A suspension is an admin's decision: only the admin path sets or lifts it, with a receipt, and the members page's Approve cannot lift it.</summary>
+public sealed class MembershipSuspensionTests : IClassFixture<TestWebApplicationFactory>
+{
+    private readonly TestWebApplicationFactory _factory;
+
+    public MembershipSuspensionTests(TestWebApplicationFactory factory) => _factory = factory;
+
+    private sealed record Seeded(Guid AdminId, Guid MemberId, Guid OrdinaryId, Guid OtherAdminId);
+
+    private async Task<Seeded> SeedAsync(MembershipStatus memberStatus = MembershipStatus.Active, bool approved = true)
+    {
+        using var scope = _factory.Services.CreateScope();
+        var db = scope.ServiceProvider.GetRequiredService<BifrostDbContext>();
+        UserAccount Account(string name, MembershipStatus status, bool admin, bool wasApproved) => new()
+        {
+            DisplayName = name,
+            BifrostIdentity = $"{name}-{Guid.NewGuid():N}"[..20],
+            NormalizedBifrostIdentity = $"{name}-{Guid.NewGuid():N}"[..20],
+            Membership = new Bifrost.Web.Domain.Membership
+            {
+                Status = status,
+                IsPlatformAdmin = admin,
+                ApprovedAtUtc = wasApproved ? DateTimeOffset.UtcNow : null
+            }
+        };
+
+        var admin = Account("admin", MembershipStatus.Active, true, true);
+        var otherAdmin = Account("admin2", MembershipStatus.Active, true, true);
+        var member = Account("member", memberStatus, false, approved);
+        var ordinary = Account("ordinary", MembershipStatus.Active, false, true);
+        db.UserAccounts.AddRange(admin, otherAdmin, member, ordinary);
+        await db.SaveChangesAsync();
+        return new Seeded(admin.Id, member.Id, ordinary.Id, otherAdmin.Id);
+    }
+
+    private async Task<T> InScopeAsync<T>(Func<IServiceProvider, BifrostDbContext, Task<T>> body)
+    {
+        using var scope = _factory.Services.CreateScope();
+        return await body(scope.ServiceProvider, scope.ServiceProvider.GetRequiredService<BifrostDbContext>());
+    }
+
+    private Task<CurrentBifrostActor> ActorAsync(Guid id) => InScopeAsync(async (_, db) =>
+        new CurrentBifrostActor(true, await db.UserAccounts.Include(x => x.Membership!).ThenInclude(x => x.RoleAssignments).SingleAsync(x => x.Id == id)));
+
+    private Task<MembershipStatus> StatusAsync(Guid id) => InScopeAsync(async (_, db) =>
+        (await db.Memberships.AsNoTracking().SingleAsync(x => x.UserAccountId == id)).Status);
+
+    private Task<List<AuditEvent>> AuditAsync(string action) => InScopeAsync((_, db) =>
+        db.AuditEvents.AsNoTracking().Where(x => x.Action == action).ToListAsync());
+
+    private Task<SuspensionOutcome> SuspendAsync(Guid by, Guid target, string? reason) => InScopeAsync(async (sp, _) =>
+        await sp.GetRequiredService<MembershipSuspensionService>().SuspendAsync(await ActorAsync(by), target, reason, CancellationToken.None));
+
+    private Task<SuspensionOutcome> UnsuspendAsync(Guid by, Guid target, string? reason) => InScopeAsync(async (sp, _) =>
+        await sp.GetRequiredService<MembershipSuspensionService>().UnsuspendAsync(await ActorAsync(by), target, reason, CancellationToken.None));
+
+    [Fact]
+    public async Task Only_a_platform_admin_can_suspend_and_the_receipt_names_who_and_why()
+    {
+        var seed = await SeedAsync();
+
+        Assert.Equal(SuspensionOutcome.NotAdmin, await SuspendAsync(seed.OrdinaryId, seed.MemberId, "because"));
+        Assert.Equal(MembershipStatus.Active, await StatusAsync(seed.MemberId));
+        Assert.Equal(SuspensionOutcome.NoReason, await SuspendAsync(seed.AdminId, seed.MemberId, "  "));
+        Assert.Equal(MembershipStatus.Active, await StatusAsync(seed.MemberId));
+
+        Assert.Equal(SuspensionOutcome.Done, await SuspendAsync(seed.AdminId, seed.MemberId, "spam in chat"));
+        Assert.Equal(MembershipStatus.Suspended, await StatusAsync(seed.MemberId));
+        var receipt = Assert.Single(await AuditAsync("membership.suspended"), x => x.Detail.Contains("spam in chat"));
+        Assert.Equal(seed.AdminId, receipt.ActorUserAccountId);
+        Assert.Contains("Active -> Suspended", receipt.Detail);
+    }
+
+    [Fact]
+    public async Task An_admin_cannot_suspend_themselves_or_another_platform_admin()
+    {
+        var seed = await SeedAsync();
+
+        Assert.Equal(SuspensionOutcome.ProtectedAccount, await SuspendAsync(seed.AdminId, seed.AdminId, "oops"));
+        Assert.Equal(SuspensionOutcome.ProtectedAccount, await SuspendAsync(seed.AdminId, seed.OtherAdminId, "coup"));
+        Assert.Equal(MembershipStatus.Active, await StatusAsync(seed.AdminId));
+        Assert.Equal(MembershipStatus.Active, await StatusAsync(seed.OtherAdminId));
+    }
+
+    [Theory]
+    [InlineData(true, MembershipStatus.Active)]
+    [InlineData(false, MembershipStatus.Authenticated)]
+    public async Task Unsuspend_works_only_by_the_admin_path_and_with_a_reason(bool approved, MembershipStatus restored)
+    {
+        var seed = await SeedAsync(MembershipStatus.Suspended, approved);
+
+        Assert.Equal(SuspensionOutcome.NotAdmin, await UnsuspendAsync(seed.OrdinaryId, seed.MemberId, "please"));
+        Assert.Equal(SuspensionOutcome.NoReason, await UnsuspendAsync(seed.AdminId, seed.MemberId, null));
+        Assert.Equal(MembershipStatus.Suspended, await StatusAsync(seed.MemberId));
+
+        Assert.Equal(SuspensionOutcome.Done, await UnsuspendAsync(seed.AdminId, seed.MemberId, "appeal upheld"));
+        Assert.Equal(restored, await StatusAsync(seed.MemberId));
+        Assert.Contains(await AuditAsync("membership.unsuspended"), x => x.Detail.Contains("appeal upheld") && x.ActorUserAccountId == seed.AdminId);
+        Assert.Equal(SuspensionOutcome.NotApplicable, await UnsuspendAsync(seed.AdminId, seed.MemberId, "again"));
+    }
+
+    [Fact]
+    public async Task The_members_page_Approve_cannot_lift_a_suspension()
+    {
+        var seed = await SeedAsync(MembershipStatus.Suspended);
+
+        var result = await InScopeAsync(async (sp, db) =>
+        {
+            var page = new IndexModel(
+                db, new FixedActor(await ActorAsync(seed.AdminId)), sp.GetRequiredService<AuditTrailService>(),
+                sp.GetRequiredService<MembershipSuspensionService>(), TimeProvider.System);
+            return await page.OnPostApproveAsync(seed.MemberId, CancellationToken.None);
+        });
+
+        Assert.IsType<RedirectToPageResult>(result);
+        Assert.Equal(MembershipStatus.Suspended, await StatusAsync(seed.MemberId));
+    }
+
+    [Fact]
+    public async Task The_members_page_suspend_handler_forbids_a_non_admin_and_changes_nothing()
+    {
+        var seed = await SeedAsync();
+
+        var result = await InScopeAsync(async (sp, db) =>
+        {
+            var page = new IndexModel(
+                db, new FixedActor(await ActorAsync(seed.OrdinaryId)), sp.GetRequiredService<AuditTrailService>(),
+                sp.GetRequiredService<MembershipSuspensionService>(), TimeProvider.System);
+            return await page.OnPostSuspendAsync(seed.MemberId, "nope", CancellationToken.None);
+        });
+
+        Assert.IsType<ForbidResult>(result);
+        Assert.Equal(MembershipStatus.Active, await StatusAsync(seed.MemberId));
+    }
+
+    private sealed class FixedActor(CurrentBifrostActor actor) : ICurrentBifrostActorAccessor
+    {
+        public Task<CurrentBifrostActor> GetAsync(CancellationToken cancellationToken) => Task.FromResult(actor);
+    }
+}