Cell C - Irritating Marketing SMS's

I repeatedly receive irritating sms's to dial in and win free data or to dial for the winning lotto numbers. I have repeatedly rep**** with STOP and OUT to no avail. I have repeatedly completed the Cell C OPT OUT form on the internet, also to no avail. I find these unsolicited sms's extremely irritating. Cell C should honour their OPT OUT form or alternatively the STOP reply. How can I stop getting these sms's?