Cell C - Over Charging New Contract

My Cell C bill has been quite high for the last three months despite me changing my contract in order to lower monthly expenses. It seems that Cell C is charging me for added benefits that I canceled on several occasions which they confirmed was done. Also previous months are appearing on current invoices which have already been paid. My new contract was supposed to be R349 per month however it is in excess of R500 per month. Calling the customer service centre does not help as they assure you it will be sorted and nothing happens....