Unable to cancel contract after numerous attempts!

I've been trying to cancel my contract with MTN and convert to prepaid since the beginning of June 2015. Finally reached a customer service consultant by the name of Carmen at your call centre on 25 June who assured me that the contract would be cancelled within 3-5 working days and converted to a prepaid number. When I queried with staff at the Menlyn Park store on 3 July, after not receiving any feedback from the consultant, I was informed that my cancellation request had not even been logged onto the system. Please assist URGENTLY as the service I'm receiving is beyond pathetic and I refuse to incur any further costs for a contract that I no longer want or can afford to maintain!