I have been trying to cancel the data contract with Vodacom for the past 3 months but not winning this one

I have been trying to cancel the data contract with Vodacom for the past 3 months but not winning this one. They keep billing me for services that I'm no longer using. Tried calling to resolve but I'm being sent from one person to another. I'm so sick and tired of Vodacom for the bad service that I'm receiving. Now I'm getting a letter of demand for the contract I canceled. I don't know who to turn to at the moment, so frustrated and exhausted. Ported my Mtn to Vodacom, I think I will go back to Mtn. Vodacom has failed me.