At this point, I'm afraid I can't think of anything immediate that you could do, as the GM was correct that since you've given the name up, anyone has the right to use it. Thing is though... If they took the name specifically to harass you by impersonation, then I doubt they'd leave it at just using a name. I'd say keep an eye on them and report them for anything malicious that they might do both on that character and on their main (if the character isn't their main already). Unfortunately, having a list of things this person has done to harass you will work more in your favor than just reporting a name change.

Also, if the name you were using before was a very common name, like a character name from a tv show or book, then it could be a completely different player just looking to make friends. (Though not so much if the name is original enough that you know it most likely points back to this person, but that would have still probably been the gm's assumption.)