Hello
Are the changes immediate?
If so then hes probably using a IFRAME or PHP Get.
You could enable 'hot linking' protection and change all of the images
that are requested from a different domain to be replace with an image
of your choice.
For an example a image that says "unofficial site, please visit
www.gamma.com."
I would contact there web host and make a formal complaint, CC the
compliant to the domain holder.
If they have any advertisements on the page that displays your site,
contact them also
Last but not least, you could add some javascript to your site that
checks the URL.
If it does not match your domain. auto redirect them back to your
site.
Good luck !
- Steven Smethurst