That's what tmobile told me to do. They deducted the text charges from my bill and I replied STOP to the next few spam texts I got. I didn't have a real bad problem. Never gave out my mobile # or used it to register for anything, but still got the odd SMS spam once in a while. But haven't received one in a long time now.
I believe I've read you can have SMS disabled by the carrier, but it is all or nothing.
Problem I was having was the SMS I was getting had a shortened number, and it didn't match the convention that was explained to me by the tmo rep. I tried googling and got nothing.
Tmobile said they had ways of dealing with the spammers, but STOP ended up working for me. I wonder if it was the same person, swithching numbers somehow, or the same two or three people.