Send them a DMCA take down notice with info about where they can properly license one of your similars.

Quote from: MarcvsTvllivs on May 18, 2012, 10:45

I just noticed that a photo of mine was taken off Flickr without a license. On the up side, at least they gave credit. But this can't be ok, can it (as in this can't be covered by Flickr's terms)? Here's the link: http://news.gather.com/viewArticle.action?articleId=281474981276875

I am also selling a slightly different version as RM through Alamy and the like.

Is there anything I can do? I usually don't bother, but then the other cases were mostly Russian websites where I figured it's not worth even contacting them.

Here's a case which may or may not be relevant. This was a well-publicised case of where Virgin Australia used a Flickr image, which had been released through the CC, probably without the photographer being aware of all the implications.
http://blog.internetcases.com/2009/01/22/no-personal-jurisdiction-over-australian-defendant-in-flickr-right-of-publicity-case
Virgin claimed that as the photo was released CC with commercial rights, they had a binding contract with the photographer. The photographer was making the case that as he had released the photo CC with restricted use, inasmuch as uses had to be credited to him, and by not posting a credit, Virgin had broken their 'agreement' with him, no contract between them existed.

However, the case was knocked out of court on a geographical technicality:
"Virgin moved to dismiss the action for lack of personal jurisdiction, arguing that it lacked minimum contacts with the forum state of Texas to satisfy constitutional due process. The court granted the motion and dismissed the case. "
So much for international law.  

Well, being a lawyer myself, I think the court was right. U.S. law has some decent tools to throw out cases that should not have gone to a particular court. That is not a bad thing. If you want to sue Virgin, sue them in Australia (or wherever their U.S. subsidiary is based), don't do it in Texas just because that's where you are likely to get better damages or a more sympathetic judge. I clerk for a commercial court judge right now and we keep getting those downright frivolous cases where companies from the U.S. sue companies from Switzerland in ****ing Hamburg, Germany, just because they can. Annoying as **** when you are the clerk who has to prepare the file, believe me .

As for the merits of that case, the photog should definitely have won. In my case here, however, I haven't even granted a CC license. The photo is under the All Rights Reserved setting on Flickr.

I would work out how much a license would cost them on alamy and send then an invoice for that amount plus an administration fee.  Might as well give them the chance to pay it before taking further action.