From experience with e-mail re-gains, I think your best course of action is to talk to the mail provider's support division with proof you own the account. They might not be able to deactivate the account,but if you claim ownership of the name and show them the problems that occured, it might push them enough to do it. Start with there and try to work it out.
If nothing comes out of it , and it really causes trouble, talk to the guy. If he's a douche, have him hacked. :muha:
Thanks for your help, unfortunately their service is only paid for now. I also have no evidence pretty much since it's been so so long, besides, I don't think they would care about some game account I'd like to have back. (I honestly blame the game for having such shitty service, not accepting my evidence list of recent activity and instead insisting on an email from the original account it was made with.)
the guy also has not replied to my emails at all, they seem to be malicious as they gained access to my (game) account through there (can't speak in details but it was pretty clear from the messages they posted that they were happy to have stolen it). any advice for such services?