My contract is with Virgin. They told me they can do nothing as its not illegal. Virgin told me to text STOP.....I did so and got immediate confirmation that "they" had stopped. Not had any since.Problem there is that you reveal the number is live, leaving yourself open for more. The reply can cost upto £10, with no guarantee they'll heed the "Stop".
I'd go with @swee'pea99's suggestion of going through the provider. They have the ability to block it in their system but choose not to.
Of course.....yours may be different to mine.
A suggestion.....google the name of the scammer and see what others have done. I did.....it is a massive but legal scam.