Do not respond to unwanted texts or emails from questionable sources. Several mobile service providers will allow you to report unwanted spam texts by simply texting/forwarding it to 7726 (or “SPAM”) to enable the providers to prevent future unwanted texts from the specific sender.

Source:
http://www.fcc.gov/guides/spam-unwanted-text-messages-and-email

More info:
http://en.wikipedia.org/wiki/Mobile_phone_spam