For the last time, it would be impossible for Steam to ban all the people who use the codes since the codes are for the AMD promo that's been going on for quite some time now, and banning all the users who use the codes would also ban a large number of users who legitimately obtained the codes, resulting in a PR nightmare.
Steam, Codemasters, and AMD will probably deactivate all the codes and remove all copies of Dirt 3 from Steam Accounts that used the codes, and then have the people who legitimately got the codes to mail in proofs of purchases to receive new codes.