The only thing I can think of doing is populating the page using javascript and interacting with the DOM. This should stop the simple method of 'right click, copy paste'.
You can also wrap each page (be it javascript, css, html etc..) in a php wrapper (for instance) which only returns true if the $_SERVER['HTTP_REFERER']is the one which is calling the selected file. That way you can't easily view the file they want to rip off.
Of course there are ways round all of these. The simplest way would be to send a polite email telling them to stop it, or you'll complain to their host...