Opt-out of an ongoing daily competition

I am extremely annoyed! Cell C has an ongoing daily R1 million draw. I have been taking part for many months and now I wish to 'opt out'. I have phoned their call centre for an opt-out option because the sms's they send do not have any 'opt-out' numbers. I was given the number to send 'STOP' to and I have sent these sms's 4/5 times and I am still getting the daily sms that I have been entered once again into the competition. This is not right. I should have an option to opt out of any daily competition that I wish,. Am a very angry customer right now...