diff --git a/public/css/nimforum.scss b/public/css/nimforum.scss
index 144f557..3bcc459 100644
--- a/public/css/nimforum.scss
+++ b/public/css/nimforum.scss
@@ -241,6 +241,10 @@ $views-color: #545d70;
 
     margin: 0;
   }
+
+  i.fas {
+    margin-right: $control-padding-x-sm;
+  }
 }
 
 .posts, .about {
@@ -384,6 +388,7 @@ $views-color: #545d70;
   .btn {
     float: right;
     margin-right: 0;
+    margin-left: $control-padding-x;
   }
 }
 
diff --git a/src/forum.nim b/src/forum.nim
index afc4eee..b7b191f 100644
--- a/src/forum.nim
+++ b/src/forum.nim
@@ -419,10 +419,28 @@ proc executeReply(c: TForumData, threadId: int, content: string,
   if rateLimitCheck(c):
     raise newForumError("You're posting too fast!")
 
+  if content.strip().len == 0:
+    raise newForumError("Message cannot be empty")
+
   if not validateRst(c, content):
     raise newForumError("Message needs to be valid RST", @["msg"])
 
-  # Verify that content can be parsed as RST.
+  # Ensure that the thread isn't locked.
+  let isLocked = getValue(
+    db,
+    sql"""
+      select isLocked from thread where id in (
+        select thread from post where id = ?
+      )
+    """,
+    threadId
+  )
+  if isLocked.len == 0:
+    raise newForumError("Thread not found.")
+
+  if isLocked == "1":
+    raise newForumError("Cannot reply to a locked thread.")
+
   let retID = insertID(
     db,
     crud(crCreate, "post", "author", "ip", "content", "thread", "replyingTo"),
@@ -630,6 +648,14 @@ proc executeUnlike(c: TForumData, postId: int) =
   # Delete the like.
   exec(db, crud(crDelete, "like"), likeId)
 
+proc executeLockState(c: TForumData, threadId: int, locked: bool) =
+  # Verify that the logged in user has the correct permissions.
+  if c.rank < Moderator:
+    raise newForumError("You cannot lock this thread.")
+
+  # Save the like.
+  exec(db, crud(crUpdate, "thread", "isLocked"), locked.int, threadId)
+
 proc executeDeletePost(c: TForumData, postId: int) =
   # Verify that this post belongs to the user.
   const postQuery = sql"""
@@ -1127,6 +1153,33 @@ routes:
     except ForumError as exc:
       resp Http400, $(%exc.data), "application/json"
 
+  post re"/(lock|unlock)":
+    createTFD()
+    if not c.loggedIn():
+      let err = PostError(
+        errorFields: @[],
+        message: "Not logged in."
+      )
+      resp Http401, $(%err), "application/json"
+
+    let formData = request.formData
+    cond "id" in formData
+
+    let threadId = getInt(formData["id"].body, -1)
+    cond threadId != -1
+
+    try:
+      case request.path
+      of "/lock":
+        executeLockState(c, threadId, true)
+      of "/unlock":
+        executeLockState(c, threadId, false)
+      else:
+        assert false
+      resp Http200, "{}", "application/json"
+    except ForumError as exc:
+      resp Http400, $(%exc.data), "application/json"
+
   post re"/delete(Post|Thread)":
     createTFD()
     if not c.loggedIn():
diff --git a/src/frontend/postbutton.nim b/src/frontend/postbutton.nim
index 5c35357..673bf62 100644
--- a/src/frontend/postbutton.nim
+++ b/src/frontend/postbutton.nim
@@ -8,7 +8,7 @@ when defined(js):
   include karax/prelude
   import karax/[kajax, kdom]
 
-  import error, karaxutils, post, user
+  import error, karaxutils, post, user, threadlist
 
   type
     PostButton* = ref object
@@ -142,4 +142,61 @@ when defined(js):
             span(class="like-count"):
               text $post.likes.len
 
-          italic(class=class({"far": not liked, "fas": liked}, "fa-heart"))
\ No newline at end of file
+          italic(class=class({"far": not liked, "fas": liked}, "fa-heart"))
+
+  type
+    LockButton* = ref object
+      error: Option[PostError]
+      loading: bool
+
+  proc newLockButton*(): LockButton =
+    LockButton()
+
+  proc onPost(httpStatus: int, response: kstring, state: LockButton,
+              thread: var Thread) =
+    postFinished:
+      thread.isLocked = not thread.isLocked
+
+  proc onLockClick(ev: Event, n: VNode, state: LockButton, thread: var Thread) =
+    if state.loading: return
+
+    state.loading = true
+    state.error = none[PostError]()
+
+    # TODO: This is a hack, karax should support this.
+    var formData = newFormData()
+    formData.append("id", $thread.id)
+    let uri =
+      if thread.isLocked:
+        makeUri("/unlock")
+      else:
+        makeUri("/lock")
+    ajaxPost(uri, @[], cast[cstring](formData),
+             (s: int, r: kstring) =>
+                onPost(s, r, state, thread))
+
+    ev.preventDefault()
+
+  proc render*(state: LockButton, thread: var Thread,
+               currentUser: Option[User]): VNode =
+    if currentUser.isNone() or
+       currentUser.get().rank < Moderator:
+      return buildHtml(tdiv())
+
+    let tooltip =
+      if state.error.isSome(): state.error.get().message
+      else: ""
+
+    result = buildHtml():
+      button(class="btn btn-secondary",
+           onClick=(e: Event, n: VNode) =>
+              onLockClick(e, n, state, thread),
+           "data-tooltip"=tooltip,
+           onmouseleave=(e: Event, n: VNode) =>
+              (state.error = none[PostError]())):
+        if thread.isLocked:
+          italic(class="fas fa-unlock-alt")
+          text " Unlock Thread"
+        else:
+          italic(class="fas fa-lock")
+          text " Lock Thread"
\ No newline at end of file
diff --git a/src/frontend/postlist.nim b/src/frontend/postlist.nim
index 84f3b7b..548d912 100644
--- a/src/frontend/postlist.nim
+++ b/src/frontend/postlist.nim
@@ -32,6 +32,7 @@ when defined(js):
       editBox: EditBox
       likeButton: LikeButton
       deleteModal: DeleteModal
+      lockButton: LockButton
 
   proc onReplyPosted(id: int)
   proc onEditPosted(id: int, content: string, subject: Option[string])
@@ -47,7 +48,8 @@ when defined(js):
       replyBox: newReplyBox(onReplyPosted),
       editBox: newEditBox(onEditPosted, onEditCancelled),
       likeButton: newLikeButton(),
-      deleteModal: newDeleteModal(onDeletePost, onDeleteThread, nil)
+      deleteModal: newDeleteModal(onDeletePost, onDeleteThread, nil),
+      lockButton: newLockButton()
     )
 
   var
@@ -364,6 +366,8 @@ when defined(js):
               italic(class="fas fa-reply")
               text " Reply"
 
+            render(state.lockButton, list.thread, currentUser)
+
           render(state.replyBox, list.thread, state.replyingTo, false)
 
           render(state.deleteModal)
\ No newline at end of file
diff --git a/src/frontend/replybox.nim b/src/frontend/replybox.nim
index 5a1e032..20e8114 100644
--- a/src/frontend/replybox.nim
+++ b/src/frontend/replybox.nim
@@ -121,7 +121,7 @@ when defined(js):
                      value=state.text)
 
           if state.error.isSome():
-            p(class="text-error",
+            span(class="text-error",
                  style=style(StyleAttr.marginTop, "0.4rem")):
               text state.error.get().message