From the manual:
//$userDeleteMsgs=1; Available from version 2.0 RC3a, this option allows users to delete their own topics and/or messages (set 1 for allowing to delete only replies, 2 - for deleting both topics and replies). This option is tied up with $userEdit option (see above) - when topic or message is expired for editing, it can not be deleted anymore. When deleting a record, PHP script makes many updates in another fields (number of posts per forum, posts per topic, posts per user etc.) - so, use this function on your own risk. Creating/deleting their topics often, users may overload your database.