Um, drag him into a town square and publicly stone him to death? Then again as mentioned a good old fashsioned hangin' , lol. Admins that was totally not funny and yes its a con.
I would have a real 'admin' from PB send a note to Yahoo abuse, to have them close down the account. Never know who just might reply to it.
Secondly, I would institute some IP logging on the forum/site, and make it more difficult for him to post. Won't stop him, but might mean he'd have to find a different system to post from. Depending on whether he is on broadband though.