Yesterday, Valve disabled approximately 20,000 Steam accounts which had been used to try to access Half-Life 2 without purchasing it. The method used was extremely easy for Valve to trace and confirm, and so there is no question that the accounts disabled were used to try and illegally obtain Half-Life 2. Accounts also may be closed due to fraudulent activity in an attempt to obtain additional products for your Steam Account. This includes Credit Card fraud, theft of accounts you do not own and using cracked versions of Valve games. If you violate the Steam Subscriber agreement your account can be permanently disabled and you will lose any products registered to those accounts.
News source: Slacknews