Cannot Cancel contract

I have been trying unsuccessfully to cancel my son's Vodacom account. In April I competed forms, and paid the amount of R1,425.99 to cancel my account. It is now 5 months later and every month I phone and every month the debit order just goes off with another R650. It is R650 x 4 = R2,600 and I am sure it will be taken in end of August again. I have now gone to a shop to migrate down to the lowest package because I have no idea what else to do.