The admin can remove them once she sees it, and they do get removed, but until she sees them, there they are. And they're listed as guests, anyone can come in and make up a name and be a "guest" so there really isnt a way to prevent it. Best bet is to just ignore it. Just like your junk email, somehow they get your email address (kind of like just dialing the phone, you may end up calling someone unlisted)and then you get all kinds of junk, even blocking it doesnt help because they just use a different email.....stupidsites123@aol...or stupidsites124@aol...they probably have hundreds/thousands to get around blockings.
Oh well...