About the Avenger thing: That wasn't, technically speaking, a hack. I talked to him afterward, and he'd more or less guessed an admin's password. That was it.
And BTW, considering how he hasn't, say, made another shot at it, I'd say give him another chance, since if he really DID have hacking skills he could very well have used them if he wanted to after being banned. If he decided not to, so much the better for you admins- perhaps, if you care so much, you shouldn't have guessable passwords. Of course, I wouldn't lay high odds on his returning even if he could, it's entirely likely now that he's found a better place to go...
On a completely other note, I do see the prob with letting him back. Still, if he can get in by guessing passwords, I or anyone else could, given a while. There's obviously a security problem there...