Yes, I think you can report the person who is stealing the art. It's against the DA rules, and I believe that a person can be temporarily or even permanently banned if they do not give credit. I'm not sure about the fine details, but I'm almost positive that there are in fact consequences for this kind of thing. If that doesn't work, then I guess the only thing left to do is making them look like a thief; you could post it in your journal to get the word around or something, I guess. Hope this helps.
Anyway...Thanks for the heads up.
-JP-
...DA needs to make this kind of thing unacceptable by doling out consequences when this kind of thing happens.
Hope this helps.