Closed forums are invisible to regular users. To administrators and moderators, the content in a closed forum is still viewable, but creating topics and comments is disabled. Moderators can only close the forum that they are assigned to manage.
Go into the forum you want to close or open.
Click on the Action bar, then select Close or Open from the drop-down menu.
You can also close/open a forum while editing the forum by selecting the Closed/Open state.