Unless they can indisputably show evidence that the game account and this account are the same (something they cannot determine without a bunch of information that more often than not needs a court order) if its just by name, that is circumstantial at best, they have no ground to bringing legal action against you, and are more than likely are simply trying to scare you. Your best bet, make sure your ign is not the same as your username here (rookie mistake) as well as disavow any connection to said account, or simply don't use the account, and create a new one. Worst they can do, is ban you.