He didn't post child porn, he talked about paedophilia in graphic detail.
I bet you there's nothing in the TOS or any laws against *describing* it. Even less so when the posts were obviously not serious in tone.
I've seen a lot worse on other forums.
Asking people to recount stories of when they have had sex with children is what he did. I believe he was asked to stop and didn't, hence the ban.
I personally wouldn't ban him for it, unless half the forum whinged about him and forced me to.
But for fuck sake, even I said worse things and never got banned for it.
I think it was rather more nipping it in the bud before it got out of hand, especially as admins aren't online all the time, it's quite a possibility that within a few hours talking about that shit, posting links etc could risk the site being cut off.
The other thing is, really if something that extreme interests him, apart from being illegal in the servers state, there are other sites to go to.
He said it was a joke, but why keep going on about the topic, when he was asked not to?