Why not just send a plain mail to all his clients:
Thanks, Tobben, that’s a superb idea! Actually hadn’t thought of that… nice!
I’d advise against this; there are a lot of pitfalls associated with directly contacting someone else’s clients. Generally, people are (or should be) suspicious of people who contact them without solicitation, and it’s reasonable for them to assume that you’d be doing so with an ulterior motive.
Who does it serve to deny this person business? He’s a charlatan, obviously, for stealing your work, but that doesn’t necessarily mean that his clients were ill-served. If you point out that their web developer is a design thief, one of two things will happen: they will respond with apathy if their own site isn’t a rip of someone elses, or they’ll look for another web designer because it is. They won’t come running to you just because you pointed it out, more likely they’ll avoid you for pointing out a costly business problem and for disrupting business as usual.