I had this exact thing happen to me last year. I tracked down the idiot's ISP and after threatening them with legal action, they stopped it. You need to find out where it originates, which might be done looking at the full headers, then go to one of the registrars, like Dotster, and use their WHOIS function. That should give you the name, address, and telephone number for the ISP. I even called them several times.
Good luck!