I have been in the cell phone industry for over 10 years with Sprint, Cingular, Altell, and T-Mobile. I also...Accepted answer
They are generated by the system. You can try to see if customer service can block them but there really isn't a way if they can't. I am sorry Vern2 comments
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you should be able to text back with the word "STOP" and it will go away, but worst case scenario you just call customer care at 1-800-937-8997 and they'll take it off