Refusal to cancel contract

I have been trying to cancel one of my contracts with Vodacom for the past few months and all I’m getting from them is that the it is not possible to do so. I finished paying for the phone a long time ago. All I want is for the line to be converted to a prepaid. I am retired and I am trying to reduce expenses, but Vodacom doesn’t care. They just keep telling me different stupid reasons and sending me from pillar to post; I don’t see why they want to force me to pay for a service I no longer need.