Interesting, given the copying of the js directly, you could use that to your advantage. Have it behave one way if the page is served up from an IP address you own, and slightly differently served from a foreign address.

Could be sublime, like adds a menu item/link to their pages that has "Web Design Services" that points back to you, to the silly "Get free copyrighted material here" and a link to some dubious content site. Or it could just break periodically and cause them great frustration until they give up and use something else.