MTN refuse to cancel my contract

MTN does not want to cancel my contract with them, the 24 months term ended in September 2017, I emailed them to cancel it and put my number on prepaid as they request contract users to write to them for contract cancellations. They never reverted, but kept on billing me even though I was not receiving any data or airtime on the sim, because they blocked it. I have been calling them and asking to be put through to retentions, but they would put me on hold for like 15 minutes until I hang up. I have tweeted them asking for the Retetions Department direct contact number, they said there is no direct line to retentions, and asked if they could assist me, I sent a private message, they never responded, I kept on sending messages, but in vain, This is reflecting badly on my credit profile as I am listed as a slow payer, because of MTN not cancelling my contract.