I usuaully say nothing, but report stolen art. Because there is no point in saying, OMGOMGOMGOMG GO 2 HELL BITCH KahbfJSLDHFBJH THT WURK IS STOLIN! LDJHGBLFJHBGDJ,. Because, of course the thief knows that. Its not like they are gonna take it down... And whenever someone comments like that, the troll is just gonna be like, U suk bekuz u think u are sooo speshul wen u arnt bukuz you cant draws a fuking stick person u sukkkk. And I don't want to waste my time on that crap.
I usually just don't do a thing, because I know the artworks have probably been reported already, and mass-reporting won't make anything go faster. Quite the opposite, usually.
Yeah, I know that. Sometimes it really is unfair, too, but I do think it is a bad idea to name the thief most of the time,because you always have those "smart" persons who need to give the troll a lecture they already know, or kids shouting stuff at the thief (sometimes getting banned themselves for harassing), and then drama starts.
True xD I say nothing because there's nothing to say: they know everything you're gonna tell them already.
"NOT YOURS! THAT IS *insertartist'sname*'S ART!" Of course they know. I mean... "OH GOD NO I THOUGHT I DREW THIS MYSELF. SORRY SORRY MY MISTAKE. WILL TAKE IT DOWN."
"Stealing art is bad " They are stealing just to get those reactions and people fall for it like chickens with no head. x) And usually, in these conversations, the troll is actually the one who sounds more mature.