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.
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.