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This function is for administrators and moderators only to lock inappropriate topics. All posts in a locked topic are still viewable, but posting replies will be disabled.
Select a topic by ticking its corresponding checkbox, then click on the Action bar when you are in a topics list;
Or, click on the Action bar when you are in a topic.
Click Lock or Unlock from the drop-down menu.
You can also lock/unlock a topic while editing the topic by ticking/unticking the Lock the topic checkbox in the Options tab.