Cant cancel a MTN contact

I have two data contracts that has finish its time and I wish to cancel them as I do not use them anymore. But that seems impossible with MTN as no branch can help me do that and the only number that I can get is 135, for al the options they answer and can help you expect for cancelations. I have called several times sometimes hanning on for more than 40 minutes without any answer but I have been called twice by an operator since they expired to renew them bur they cant help my cancel them. Please help what must I do??