Websites are HTML, CSS, and JavaScript, all of which is open text and easily copied. You shouldn't have any illusions that your design and code is copied and reused somewhere else.
In this case, the websites that are using the design aren't competitors and aren't taking away business from the original developer. What are they expecting to get out of it? An additional revenue stream of HTML templates?
I agree it's a problem if they are hot-linking to assets on someone else's server, but that's a problem in that it's consuming someone's bandwidth and server resources.
I say, call it flattery and be proud that you made a design that other people want to copy. If you're really paranoid (I don't see why), ask for attribution like "Site design by ..." at the bottom of the page.