Art thefts, be they here on dA, on Ebay, Facebook, have small online shops and whatnot are an annoying problem since many years.
So what should you do to prevent that someone else uses your work, your talent and time without your consent to gain money, fame, "faves" or whatever else by copying it and showing it without crediting you at least properly?
1. Stay calm, don't overreact. Look what kind of people or individuals used your art and for what reasons. Try to think strategically instead of writing an angry comment on someone's profile or facebook page in caps. That leads usually to nothing.
2. Look up if you can contact any copyright, anti-piracy teams by the website where something copied/stlen was shown/offered. I saw a case today as example where someome tried to sell t-shirts and mugs of a fan art of Groot from Guardians of the Galaxy ..without crediting the artist and by erasing the watermark and signature of the artist.
The person tried to sell it via a small face