VODACOM FROM HELL

I had a contract with vodacom before my contract was die for an upgrade I called to cancel the contract, the agent offered me airtime and data for a lower amount as he said I was a long serving client, little did I know was a call for disaster, he informed me I can cancel at anytime, today I called them to cancel the contract and they camw with the same tactics to try to keep from cancelling, when I insisted then they tell me there will penalties… Please tell me what penalties should I pay on a contract I can cancel anytime, I only receive data and airtime so what costs did they incur if I say I dont want their airtime and data anymore that is of no use to me. Does vodacom leave on milking their customers or are they vultures that enjoy on sucking peoples blood. I want this contract cancelled.