If you normally sell those graphics, then you have a better chance of succeeding at taking them to court. Generally, you have to prove one of two things: loss of reputation and/or loss of income.
If they're free for the asking, as long as the copyright notice remains, then it will be hard to actually win anything in court, and it'll cost you a bundle to do it.
Or... you could add a very small "©2003, andrea " bit of text in a corner of each graphic and make it impossible to steal them without giving you credit. Of course if the graphics are icons or smileys, that would be hard to do.

You could email them until you're blue in the face, but is it worth the frustration? Maybe if the graphics would normally have been sold, but otherwise... I don't think so.
Good luck, Michele