Cancelling Data Contract

I have been struggling to cancel my data contract that was supposed to end at, end of March. I called in February to inform them that I will not be upgrading, I want to cancel the contract end of March. They told me to call in March. I called again in March to again inform them that I do not want to upgrade, I want to cancel. To this day I am still struggling with getting them to cancel that stupid contract. I have called and emailed with no luck. They keep on adding the data and I have told them that I will not pay them. I have been with Vodacom for over 10 years and this service that I am receiving now is of utmost disappointment. Jerr!!!!!