dunno if this could work but im pretty sure you could, take back the account and then send the guy a email from blizzard that it was banned for account sharing or trading something like that also when you take it back NEVER log on to it as he can just check the armory and see that it was updated. Then transfer servers or do a name change and he will not know that you still have it. To be safer you could xfer off the acct then ban the orig acct legit.