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So what's the problem James? Seems to me you just picked up another sales outlet.
This is basically a copyright issue, so enforcement and retribution will be mostly up to you and your lawyer. In the meantime, be sure to have a witness document the source code and perhaps take a screen shot. You can't do it because anyone could easily fake it.
I've had this happen too, as long as you just want them to take it down, then send e-mail to the site owner. On the occasions it has happened to me, most respond with profuse apology and change it quickly, like it was an accident. The most often used line is - "You can be sure I'll speak to my webmaster, I had no idea he did that" - yeah ok.