FAQ: How to get rid of mobile spam

AT&T: Customers can block text messages or calls from a specific phone number on its Web site here, as well as restrict the sources of e-mail that reach your phone on this site. Customers can also reply to text messages by typing in "BLOCK" or "STOP" to prevent future messages from that sender.

Sprint: Sprint wants customers to call customer service to report all spam messages so the company can modify its spam-filtering technology to block the phone numbers that are sending it, said Sprint spokesman John Taylor. Customers should not reply to the messages, otherwise it verifies to the spammer that the phone number is valid, he said.

T-Mobile: Postpaid and FlexPay customers can create their own filters and block chargeable text messages, MMS (multi-media service) messages, instant messages, and e-mail from being sent to their phones by calling customer service.

Verizon: Customers can log into the site and sign up for Usage Controls ($4.99 a month) that allow them to block certain numbers from calling or sending text messages to the phone. And if customers text only with a few people they can create an alias address here for free and receive only text messages sent to that address, said Verizon spokeswoman Debra Lewis.

Source: http://www.cnn.com/2009/TECH/07/09/cnet.cell.phones.spam.messages/index.html