MTN will not cancel expired contract

My contract with MTN expired on the 7th May 2015. I called the customer service call centre to change the number on the 7th May to Prepaid in March but was told I need to call back on the 7th April even though I held on for 25 minutes waiting for someone to answer the phone. I sent an email on the 7th April and received a reference number so assuming that it would be changed to Prepaid left it at that. When I tried to port to Vodacom the middle of May the port was rejected as the number was still on contract and I had to get a new number. I have made more calls and sent further emails and the contract has still not been cancelled. I will be cancelling the debit order to prevent MTN from *******ly taking money from my account.