Add a comment (or attribute or JS with a string literal) to your HTML that contains IP address of whoever requested the page. Obscure it somehow so it's not obvious that the HTML contains the IP address. Then check source code of the copy, and you'll see who requested it. You can then go after that IP.

BTW: if they're removing/replacing domain name of your site, try obscuring it with HTML entities. This may dodge simple find'n'replace.