i usually take a screenshot of the two pieces next to each other... and then i contact the original artist and talk it over with them before i move on to reporting it to the admins - it usually goes better if the original artist does it.
I'd check with both artists to make sure they are both aware. Could be innocent (a collab for instance) but may not be. If it ISN'T innocent then give the thief a warning and advise against them doing it again. If they persist - report the blighter!
I think the "Flame them in a journal" is an under represented choice here. I get using people's art for avatars or stamps or whatever. HOWEVER, claiming it's your art is not okay.
People need to understand that this is bad not because of some administrative reason. It's not against the rules, it's straight up unethical.
I'm not saying it's the best thing to do, and it's definitely not what I would do. However, I don't think it's the WRONG thing to do. If somebody's stealing your work, and that was the recourse you took, you'll get no judgment from me.
That said, when people use other's art for things like signatures and whatnot, I know of some artists who give WAY too much of a shit. Attacking someone over that seems silly.
Now that I think about it actually, maybe the journal idea is only okay if it's your own art that's being stolen...
if I know the artist id tell them, its not just here many facebook apps are also using DA artworks on their pages so i try to inform the artist when ever I can other wise I report it to some admin or the other