Yes that's what happens here at the moment. The member asks to leave and then admin uses the term 'guest' for all that persons past posts. Their name remains though. Thanks for replyingGuest status generally doesn't allow any postings, I have never reverted someone to guest but I doubt the posts would remain. You can change the individuals screen name and then delete all the information boxes, which leaves the posts under a new name (you could use 'Guest' or 'Past Member'). Neither of those could be carried out by the member as it requires admin status to access the database, but it would achieve what you ae asking for.
