As I understand this request is totally same functionality what Facebook groups are using. An user can delete own post when ever wanted so. If deleted one started a topic/thread then every posts will be deleted too. If deleted one is a post that has replies, those replies will be deleted too.
Well, admin’s choise if such move will be allowed. Not asked, but in Facebook that is heavily hated becuse then one person can delete anothers work at same. In my FB-groups such action will lead immident kick out of that group.