Cell C Relentlessly spams its own customers!

I am constantly getting spam SMSes from Cell C flogging all kinds of services I don't want and will never want. Every single one comes from a different (randomly generated?) number, and so no amount of number blocking or replies begging them to "STOP" seems to prevents the mindless spam. I am getting very tired of this, as these waste-of-space messages are burying actual important and legitimate messages which I need to see!