Not happy with Vodacom as I can't cancel my contract

I tried to cancel my data contract with vodacom at the end of October, and I got a sms saying that I will receive an sms or email with the cancellation date. By the 18th of November I still hadn't received anything. Then I phoned again, and they said I have to pay a penalty because I had recently upgraded, which I didn't do. So they escalated the matter and said it would take 24-48 hours to receive a response. It has now been a week, and I still have no response. The service is unacceptable, as this means I will be paying for an extra month, that I don't want.