Impossible to Cancel my contract with Vodacom

I ported my number to MTN from Vodacom. I called them on the 30th of November to cancel my contract and they told me to call them after the 5th of December because they are busy with billing. I have called them today and the agent can't cancel, she is telling me to call next month and I don't understand why?? Vodacom is billing me for service that I don't use or need I want to cancel and even pay whatever penalty they charge . Their call center is useless and rude, they even cut my call while I was talking