I'm sorry to hear this has happened to you! I'm a regular on flickr.com and see this happening A LOT, but it never gets less annoying! It's far too easy to right click and steal someone's photo - and even if you disable right click saving, there are many other ways they can steal it.
I hope they take the photo down. I don't mean to bear bad news, but if it's not copyrighted, they and anyone else can do whatever they want.
Watermarking is the next best thing and can be done in most photo editing programs.