Hi, sorry to hear about your problem. Do you know what country he is in? what country are you in? much easier to deal with if you're both in in the same country.
You may be able to find out where he is (maybe even his physical address) by searching uk2.com. Do a search for his domain name, it will tell you it's registered but will give you a link for the registrant's details. It's not too difficult to falsify or hide the info but most of the time it's ok.
If you can get his (or his service provider's) address and you're in the same country, see an IP (Intellectual Property) lawyer and get them to send an officious looking 'cease and desist' letter - this is enough to scare 99% of people out of doing whatever they are doing.
There should be a way of finding out who his provider is from the nameservers, although I'm not sure how - someone here can hopefully help you with that.
I suppose it depends on how much money it's worth to you but a cease and desist letter sent by a lawyer direct to him might still do the trick. It would certainly 'put your stake in the ground' from a legal perspective - i.e. he can't argue that he didn't know it was wrong once he's recieved it. I would guess you could get one sent for £50-£100 maybe less if you can find a local solicitor who knows about IP