Do not respond to unwanted texts or emails from questionable sources. Several mobile service providers (including AT&T) 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!

For further information and citations, click on the "Info" link, below:

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

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.
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Please reply to this post, and let us know if you tried the suggestion, and if it did or did not work.