The problem is that, if this person sees that this thread has been made, and is aware, they can get everyone who has a link to their DA in trouble, because it is considered bullying if there's suddenly lots of activity and rude messages. That's the deviantart way, stolen art is only stolen if its physical, and as long as someone is rude to you, they get banned and you can get away clean. Thats why it just isn't safe to post something big like this to the public. But, hey, it's none of my business.
I have looked and there are not too rude messages, only those that tell them that they stole art and rightfully demand it all taken down. In my book, that is not bullying. I would call it bullying if they attacked their person and called them names, along with threats to their life.
I mean, do you call intercepting a thief that stole the purse of an old lady assault and bullying?
People there are merely fighting back and making it clear that they do not like what they are doing. However, I do agree that there should be physical proof of the theft. So sadly, OP can only truly fight back effectively, if they have the pictures posted somewhere with an earlier date than the theft. Otherwise, sadly, they have nothing in their hand and the only way to stop the thief is to make this public and make people aware of their continued theft.
This way, even if they post pictures on their profile and claim it as their own, people will already know it was stolen. This usually stops a thief in a few months.