MTN charging me for services not used

I have an mtn data contract that has expired in 2013. I had requested that my account be changed to prepaid but was told I had an outstanding amount which I had to pay. I paid the outstanding amount only to find that I am still being billed even though I was not using any of their data. I notified the consultants via email who then said it was an error in the system and will be resolved. After a while I was still being charged for a service I was not using. I have to date contacted 3 consultants who told me the matter will be arranged only to find my account is still being charged. I have reached a point where if this matter is not resolved I will take my own legal actions as this matter is affecting my credit record. What I need from MTN is to once and fall close my account and write off my so called money I owe them. I have prove, in a form of email, that all of this transpired with the 3 operators I communicated with.