All you really need to do is make a new account, buy Arma 2 OA (make sure you don't install it again or run it again), hit library, Arma 2 OA and then under 'Links' hit CD Key. Copy that CD key and use Darky Key Changer to change your banned one, to your unbanned one. Feel free to log back into your other Steam account however, just launch with DayZ Commander from now on. If you through the comments, I believe someone posted a method on how to launch with Steam without your CD key changing back but just launching with Commander is a lot easier.