Account billed whilst I no longer have a contract

I had a 24 months data contract with Cell C that came to an end in October 2015 and when I contacted them to advise them that I want to cancel the contract and convert the SIM card to prepaid, I was told that someone renewed the contract and the matter is sent to ***** department a case number was sent and no response returned and I am still billed even larger amount of money as I was paying R190 for the data contract and now I am billed R638.99.<br> <br> Thanks Thabang