Thanks for the replies! He stopped for now but of course he will be back some day. We've got a payed membergroup that we could allow banning accounts with less than 50 posts. It's actually a pretty good idea. If someone would abuse it they would lose $50 so I don't think anyone would want that.
With over 300 payed accounts this is a pretty big moderating team

Of course this system should be very safe and tested waterproof as things could go terribly wrong if there are any security flaws...