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.

I think it works the following: Assuming the proxy has a different IP pointing to it's client, by inserting the IP it uses to connect to the original server into the HTTP reply (HTML/body code), it can be exposed to the OP. However, since he seems to have access logs and seems to understand the proxy requests pretty well, I wonder how it actually helps.

That's clever, and I just understand after a while.

Now let's say that your website will show the ip of whoever visit it, in one of textbox. When you access it it shows your ip. When the proxy sever access it it shows the proxy sever's ip. When you access the proxy site, the proxy site will access your site, having their ip on one of the text box, then return the page with their ip to you.

The more advanced method is to encrypt the ip and put it hidden somewhere, on later for you to decrypt it, get the ip and black list them.