On a "fight fire with fire" note, see what you can find about this site. Do a whois lookup (probably won't find much, if they know what they're doing). You may be able to find their hosting company and contact them directly. Also see if you can find where the site is hosted geographically. This will give you an advantage of knowledge should you need to take any legal action (a host in Utah is easier to fight legally than an off-shore one). I also like the suggestion about creating a script to bombard their servers with false registrations - maybe as a last resort thing...but maybe not. :)
Also, would you mind sharing the sites with us (both yours and the bogus one)? I would personally like to see how this site is trying to phish information from your visitors.
if not report it to the web host they are on.