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Locking a forum will prevent all members from creating new topics and posting replies in this forum. All topics in a locked forum will be locked also, but their content is still viewable. Moderators can only lock/unlock the forum that they manage.
Go into the forum.
Click on the Action bar, then click Lock/Unlock from the drop-down menu.
You can also lock/unlock a forum while editing the forum by selecting the Locked/Unlocked status.