If there are no posts made for 35 days, forums get deleted automatically. So, if you do not want your forum deleted, make sure to post at least once within 35 days. Once a forum is deleted, you cannot get it back!
If you DO want your forum deleted, lock all the areas so no one can post, then just abandon the forum. It will then be auto-deleted after 35 days with no posts.
ForumUp World-Wide Administrator