Contract not cancelled

My contract expired in August 2015, but I kept on receving invoices. I phoned on 19 October to find out how to cancel the contract. I was told that it will be cancelled. I received an e-mail on 21 October, confirming the cancellation. It was sent by Ncebakazi Filita MTN retentions. Reference number <br> ***6. Today I received another invoice. I phoned the call centre and was informed that the contract was not cancelled, because I didn't give a reason for cancelling. Totally unexeptable!