The other kind of problem is if the website is not really proxied but rather dumped, patched and re-served. In such case the only option (if JavaScript frontend redirect doesn't work) is blocking by IP the dumping server.

To identify IPs, as pointed in the root comment of this thread, you can create a one-pixel link to a dummy page, which dumping software would visit, but a human wouldn't. So you will see who visited that specific page and block those IPs for good.