Vodacom just takes money because they make it impossible to cancel a contract

Last year I cancelled my data contract with Vodacom which came to an end. It was never cancelled on their side and they continued billing me. In January I contacted them again and after 45 minutes of my airtime and me convincing the other person on the line I just want to cancel the contract and no I do not want an upgrade or to keep the number I just want it cancelled. I was told my final payment would go off at the end of February. I have just received my latest statement and it appears I am now going to be be billed R46,29 for the rest of my life because this number is now apparently reduced to a pre-paid. If I dial the cancellation number I cannot get past the step of inserting the cell number because its "pre-paid" If I dial the customer are there is no option for me to select pre-paid cancellation. So now it appears that Vodacom just get away with charging people monthly amounts because we have no way of cancelling it.