Cannot cancel my data contract

I have phoned Vodacome 3 times to cancel my data contract. The first time was on the 26th of July, of which Vodacom has no record, only that they offered me a new subscription. In September I was informed that it had not been cancelled by an agent who called me to add on to my existing data package (which was supposed to have been cancelled). I phoned again on the 12th of September to cancel, and then again on the 13th of September. I was reassured that it is cancelled and that I will not be billed anymore. But I was billed again now in October. Vodacom, how do you suggest that I cancel my data contract?