Once you're banned on your steam key you can't run through steam anymore on that account or it will just change your key back to the banned one. You must buy a new key from either steam ($30) or someone selling them on the forums (DayZ Selling / Trading / Buying - MPGH - MultiPlayer Game Hacking - Hacks, Cheats, Trainers, Games, Downloads) and change it with a CD-key changer that they will most likely provide, or you can find one on the forums. No need to delete registry files.