Unable To Cancel Contract

On the 28th of September 2017 I emailed MTN Customer Care with a request to either convert my account to a pre-paid account or if that is not possible to cancel the account as it is older than 24 months. I did this because I got tired of being billed for content charges every month on a data Sim which I did not ask for or authorize. On the 12th of October 2017 I received an email from Retentions indicating that I will take between 2 and 24 hours to convert my account to pre-paid. I was supp**** with ********** 2 as reference number. On the 16th of October 2017 I emailed Retentions requesting them to proceed with the conversion as I have obtained a new internet service provider. On the 23rd of October 2017 I emailed Customer Care and Retentions asking for confirmation that my account was converted. On the 26th of October 2017 I had to email again to ask for confirmation of the conversion but as yet no reply or confirmation was received just an expectation to pay an excessive account which exceeds to set credit limit. How do I proceed in this matter to get my account cancelled as I am not willing to pay any money to MTN until all my issues are resolved.