Incorrect Billing Cannot Be Resolved By Vodacom!

My 24 month cellphone contract was expiring end September 2015. I called the Vodacom Customer Care line end August 2015 to confirm that I will not be renewing the contract. This was actioned correctly and I was converted to Prepaid on 30 September 2015. When I called to cancel the contract I was informed that my last contract premium amount would be debited from my account on 01 September for the free minutes, SMS and data I am allocated monthly as per contract rules