Hello Kohpong,
Thank you for contacting Steam Support.
Attempting credit card fraud is a very serious issue and is prosecutable by law. Normally, this activity would result in permanent restrictions on your account.
However, as none of the attempted purchases were successful, we have made a one-time exception and modified the account lock to expire in 30 days.
You will not receive another warning regarding this behavior. Any further activity of this type, on any account, will cause all of your Steam accounts to be permanently locked.
While the account is restricted, you will be unable to make purchases, trade, receive gifts, or participate in the Steam Community Market. This lock will not be removed prior to the time specified above.
Valve reserves the right to refer the matter to the appropriate authorities.