Once they are banned (which may happen later), the ratings loss to you is gone. But even if that wasn't the case, the grief they have caused you (as evidenced in your postings) is much greater than what would have happened if you just had resigned and blacklisted.
Please, do not make your emotional well-being dependent on some rating number in an online game, that is not worth it. I understand that you don't want your rating to suffer, but the effect of just one game is usually negliable. You fell exactly into that player's trap. They want to see you suffer in the game, they don't care about your 0.2 level drop after resigning.
Because the game is free and an account just requires an email, it's fundamentally hard to ban players, it's just easy to ban accounts. By all occurances, Johnsy is not the first account that player used, for example.