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 forwarding them to 7726 (or “spam”) to enable the providers to prevent future unwanted texts from the specific sender.
TIP:
Put 7726 in your contact list, so you can easily forward those unwanted texts!
More info:
http://en.wikipedia.org/wiki/Mobile_phone_spam
http://blog.usa.gov/post/89152807112/protect-yourself-from-text-message-spam
However, in many cases, the text origin, as seen on your phone, is faked. The internet domain does not exist, or the phone number may have been spoofed.