SMS like that are most likely geing generated from a mass SMSer server/website/app. If they are advertisements, call your provider and have them block spam SMSs. If it's someone who's harrassing you, you'll most likely need to just change your phone number and NOT give it out again. They are most likely getting on Sprint's website and generating the SMS to your phone number that way... anyone can do this and does not require a return number or even a valid account of any kind.