To lock or unlock a thread, you need either the moderator or operator role.

Locked threads cannot be posted in by anyone. This is useful for certain announcements, old threads, discussions that have been completed or to prevent issues from getting out of hand.

To lock or unlock a thread, browse to the thread. In the top right corner next to the thread's title, the edit button takes you to the edit page.

If the checkbox for locking a thread does not appear, you may not be logged in or may not have moderator or operator priviledge. Also note that you can only lock a thread by editing it. You cannot set it to locked during creation.