MTN absurd incorrect bills

My wife has a contract at MTN that costs about R650 per month and she purchases an extra 2GB of data bundle extra each month. She got charged R2100, R3000, and R1200 in sequence in the last 3 months. We contacted them to find out what is going on and they told us its data charges, the one bill displays R1800 for data. She works in a WIFI area and her data bundles displays as sufficient when viewed. According to this bill she used up about 1,8GB of out of bundle data above her purchased 2GB of data and her WIFI access. This is not possible under normal use. MTN didn't help us to resolve the issue and also didn't limit the data after it has been requested. The next month we received a bill of R2100. We again looked at the bill and realized she have been charges for obscure amounts of data and other unknown items. Today she received an sms claiming her data is depleted and she checked her balance and it displays 1,5GB available. I call on MTN to pay back the wrongfully allocated costs and cancel the contract as they are not keeping their end of the contract and cannot be trusted.