MTN keeping me hostage

My contact expired in December 2015. I proactively called MTN in November to advise them of my wish to cancel the contract and be converted to prepaid. To date that hasn't been done. All I get from their *********** call centre is frustration. From agents to Team Leaders. I get told that my request has been escalated and nothing happens thereafter. When you complain on Twitter, you deal with different people every single time and have to repeat your complaint over and over. MTN has really lost it. After that conversion, I am leaving your sorry network and never to come back. I can't live with such *******. Plus lately I've been experiencing huge inconvenience. When I make a call, I hear a message that my airtime is exhausted even though I have airtime. When I call the useless call centre I get told to reboot my phone because that is possible to occur. *******! Sickening!