MTN ACCOUNT ISSUES

I had an MTN account and I paid the amount in full. Last year I needed a Wi-Fi router and i wasn't assisted as they say I have an 85c due to me and they unable to concel. It bufeddled me as they are the store that deals with such things, they were supposed to cancel or something and close my account. Now I want to get a new phone and I'm unable to because of the account that they cannot close. This phone is required immediately for work and I'm being delayed.