We're hitting up against this issue as well. We've restricted permission to create ad-hoc rooms (after tons got wrongly created), but there doesn't appear to be a mechanism to delete those ad-hoc rooms (even if it's not associated with an event, even if the person trying to do the deleting is the person who created the room, and in the context where they created it). Should there be some sort of clean up process running via cron which removes these now unneeded ad-hoc rooms?