Cancellation of 1 x contract and 1 x contract reverts to pre-paid

I cancelled one of my contracts and the other will go on prepaid on 10/10/2017, cancellation date 07/09/2017, Ref no: ********** ********** 06. On 01/10/2017. I received my full data and airtime packages as normal???? I was suppose to receive a pro-rate amount till 10/10/2017. So on phoning Cell C today I was told that I will pay for the full month and will forfeit all unused airtime and data on the 10/10/2017. I have given them a months notice as I should, I will not pay for a service that I will not received from 10/10/2017. Cell C must bill me for what I have used and not ***** from me as they are planning to do from 10/10/2017.