Cell C wrongfully billing me for more than a year

I had a cell C contract which ended in 2014. I called a month before to notify cell c that I dont want an upgrade but want to convert to prepaid. Cell C debited my account the following month with over R2800. I enquired about this amount and was told its for data. I requested a report showing detail of how I spent the data, until this day I have not received any report from Cell C, instead I get debited every month with R5000. Instead of getting a report Cell C sent an email that they have credited my account with a small fee which doesnt even reach R100. I keep reversing this amount but it is costing me a lot of money for more than a year now doing this. I will not pay this amount until I see proof of the R5000.