How to stop SMS spam from mobile operator?
I stopped this sort of spam from Three by forwarding every text I received from them to their own spam reporting number, 37726.

You will get an automated reply asking for the number that sent you the text.  As you don't get a number from the spam, just use Three's own marketing number, 0800 033 8002.

It didn't work immediately but they did take the hint after a couple of weeks.