Avon Justine sends ******* spam

This company has been sending me spam via SMS. I have never been involved with this company in any way - I don't know how they got my number. The messages do not contain an option to opt out - I don't think that's legal. Each time, I block the number but, of course, the messages keep coming from different numbers. I have had numerous conversations with call-centre agents. They are very helpful at first, and they tell me they will look into the problem, but that's as far as it goes, and the spam keeps on coming. Why is it so difficult to remove my number from their list? This is unsolicited direct marketing which, according to the Protection of Personal Information Act, is *******.