Another designer, if the person has registered your trademark in a domain name using .biz, .com, .info, .name, .net, or .org then I would remind him of the ICANN Uniform Domain Name Dispute Resolution Policy (http://www.icann.org/udrp/udrp.htm) of which he will have agreed to when purchasing his domain name.
For your info and to give you an idea of how this works, I had a friend who had registered a name with the Disney trademark, even though I warned him against using the name he would not listen and has since had his domain name transferred from him after Disney pursued the case with the National Arbitration Forum (http://www.arb-forum.com). My friend didn't respond to Disney and 2 months later the arb-forum simply ordered that Godaddy (the registrar) transfer the name to Disney in accordance with the ICANN UDRP policy.
Disney based there case on the grounds that the domain name incorporated their trademark, respondent (my friend) has no legitimate interest in the domain name and that the domain name is used in bad faith. (as mentioned in an above post, so I guess this is all you need)
After contacting via e-mail, and having not responded to them, Disney decided to send 100 pages of documentation outlining their course of action to his home address. lol
Maybe you could use some of the above info to create an e-mail warning him. If that doesn't work then I'd get some legal representation.