Summary. AT&T Wireless has a help page with instructions on what can be done to stop unwanted SMS messages. In summary, you can forward any received messages to 7726 (SPAM). This will alert AT&T of the spammer. You can also setup a MyMessages account that will allow you to block messages from known spammers. AT&T advises that you send STOP as a reply to any unwanted messages from short code sources (such as 770-50).
One recent source of widespread spam has been the sender MobChance.com (short code of 770-50) who has been sending short trivia message concluding with the words, “For HELP call 18888906150.” Some people claim to have been charged a fee after calling that number.