Oh dear :c it's irritating when people steal art, and that's a very blatant example... they didn't even try to cover it up with a trace!
And switching viewpoints for a second... I hate to be the devil's advocate here, but what if they're just some little kid who doesn't know how art copyright works? I've encountered lots of them in the past, they seem pretty stupid, sure, but there's a chance they just don't know how things work yet. Wouldn't it be better to just tell them off politely? You'll save yourself a little anger too, perhaps?
I just don't like seeing people leap at the throats of first-time offenders, especially if they're young and new... it can scare them off of art sites forever. They just need to learn the rules and chances are they'll quit whatever stupid crap they're pulling. :c
I have nothing against you, quite frankly I think you're a great friend, and I don't view art theft as in any way acceptable. But when we're young and stupid, we deserve a second chance, wouldn't you think?
I've dealt with art thief/tracing before with my art on occasion, and I find things can be cleared up with a bit of civil discourse. If they've refused to be civil that's another matter entirely though, but at least give people one shot to redeem themselves.

Yea, I do admit I went quite a bit overboard... I talked to my mom about it and she said the same thing.. I won't do that again (looks like I'M the little kid here >.<)
True, it is terrible for art thieves to steal, but I can see where you and my mom are coming from XP

She also said that some adults find it perfectly acceptable, including on something called "Pinterest" where they don't seem to know the meaning of "theif" >.<