Constant spam SMS even after opting out

MTN's Quiz4Moola sms which i get ever Saturday and Wednesday without fail contains the option to opt out. I have tried this option three times already. Each time i get another SMS saying \oh no, you've stopped playing! you have points, get them back to win bla bla etc\". This would make one thing that is it but no, come the next Wednesday or Saturday i get another SMS just like the rest of them. With all the ***** accusations etc going on with this company one would think they could try not to annoy everyone that is still stuck using them until the contracts run out."