Terrible service

I have had a contract with vodacom,of which I have settled early in September last year,as I did not want the contract.I have paid the settlement fee,and vodacom confirmed it is now over.But last month they have deducted from my account (I have put a stop order now).I have also resent the original cancellation forms to vodacom (thrice),but no response.I do not want the contract....I just want it as a pre-paid as agreed.Can someone at vodacom please assist,as i am getting NO joy from your cancellation department