After checking today, it looks like all he's done is mess with people's sign. Apart from a minor annoyance, it's hardly a banning offence and not really griefing. Until we can prove that he's responsible for damaging people's homes, and stealing from chests then we can't really kick him off the server. Plus, he's pretty much giving the game away what he's done when he puts his own name on the signs!
But like everyone, if you're caught or seen by admins or ops damaging or stealing from other users, then your ass gets banned.
In the meantime, we're looking at installing a mod that will protect chests so that only the person who owns it can access it. I'll let you know as soon as I do how it works.
On a side note, I fucking love that texture pack you're using - any link to the download file?