Vodacom refuse to realease me from contarct that has expired

My contract end in October , Every time I phone to advise them that I a m not renew contract . they told me that they will give me les expensive contract .This occurred until month end and they forward me airtime for another month . one of reasons I want to cancel my contract is that air time and money paid was increased and I request the telephonic record of the conversation that I have told them to increase i was taken to pillar and post. I have been with Vodacom for more than 15 years .