LOL, no you can't unjoin, think of an artist that has been permantly banned, do the search members...you'll find them there...my guess is that way Rendo can say they have such and such many users, when I bet the real number is about half with all the multiple IDs, banned members and so on.  I think if a user isn't active within a year they should probably dump the ID 1) If there's a gallery attached it'll be a good way to clean out stuff and save webspace
2) Most of the good usernames are attached to those people who aren't here anymore