Contract expired but billing continues

My contract with Vodacom on *** expired at the end of April 2015. Telesales phoned me to have it renewed. At first I agreed to extend it for another 24 months on different terms but because I was dissatisfied with Vodacom's bad service, I lodged a report on Hello Peter at the beginning of May 2015 to cancel the contract. A consultant phoned me to inform me that Vodacom agrees to the cancellation of the contract. I returned the tablet together with the sim card via a courier service. Nevertheless Vodacom keeps on billing me for this contract every month since the end of April even though I have not used it.. I request Vodacom Telesales to contact me and send written confirmation that the contract is cancelled, backdated from end April and all amounts deducted from my bank account in respect of this contract since 1 May 2015 be refunded.