well, I hope you consider changing the license of GUIminer to something that makes it stay free software, but prohibits malicious redistribution of that code.
At least you could then file a lawsuit against them, for copyright issues. Also, be sure to get a whois output for their domain and report their doings to the registrar. then do a whois on the ip-address where the site is hosted and write some mail to their hoster.(if domain registrar and hoster are not the same company). These usually react fast to complaints. They react even faster to DMCA complaints(in case you really change your license)