Still receiving marketing messages from MTN, despite complaint

I still have the same problem with MTN sending me marketing sms's despite my complaint on this site a while back. There is no number - it just says MTN and although I opted out a few times by sms'ing STOP to 30246, two days later I'll get a marketing sms directly from MTN again. Nobody has contacted me yet from MTN. Why give customers the option to opt out, if the messages keep on coming? How can I opt out permanently from marketing messages?