Poor Service/Billing when contract expired

I had a Cell C contract which was expiring on the 6/5/2015 after 24 months. I had informed them by email that I would continue on a month to month basis for which I have email proof plus I have a reference number from the call centre. However they disconnected my service and switch to prepaid on on 17/5/15. I had not used my phone from the 27/4/15 as I had been out of the country. When I contacted the call centre they stated my contract is expired and they don't know why I was changed to prepaid. Then at the end of the month of May 2015 they debited my bank account with R409. I contacted the call centre several times and given reference numbers with no response from Cell C. This is ****************. How can I be billed for for a service I did not use over and above the fact when I requested to stay on postpaid they switch me to prepaid. When you go into the Cell C store, they say there is nothing they can do and that I must just instruct the bank to reverse. This is theft and poor service. Dont ever take a contract will Cell C unless you want to go prematurely grey.