you dont really have to do anything, if they continue blatantly stealing your art you can make a collection of side-by-sides with yours & their copy, along with the dates from when they were originally posted, just about every artist i know have been through this before and it always ends the same, they make a thread displaying the copied pieces and the thief gets enormous backlash until they simply delete their profile all together
my personal opinion is that you shouldnt care too much, great artist are sort of bound of having their work stolen or copied and with that sort of blatant proof they always remain protected, they mightve fooled a few people but thats their problem, it always sorts itself out in the end