MTN ignoring Opt Out STOP Messages

I have repeatedly sent MTN the \STOP"Opt out message to stop them sending me sms messages about MTNQuizforMOOLA. They send me a standard message of acknowledgement and then a day later- (although there have been times when it was minutes later) they send me another one. I'm so annoyed with them for sending me so much spam that I'm thinking of going with another network after being with MTN for 15 years. """