Continuing the contract upon requesting not to be renewed

My contract with telkom was ending in June 2017. I did send the e-mail in May 2017 to make them aware that i would not be renewing the contract. They renewed the contract on their own. I called them, emailed them and got their staff from telkom shop to send the cancellation forms , but still the contract has not been cancelled. My credit record has been clean but telkom now got me blacklisted because I eventually stopped to pay as they were not cancelling my contract. I have had the telephone and internet service with telkom for about twelve years. I have not skipped paying for that period, but they are treating me badly. Telkom has no manager to talk to, only call centre staff.