Incompetency that leads to financial loss

Is there any way I can rate a zero because MTN is just taking me for one big ride. I had a call from one of their representatives who told me that instead of cancellation I can get a SIM only deal for 89 per month, bear in mind I've been with them since I can remember. I decided to go ahead with it and she advised I would get 4gb daytime and 4gb nighttime. That sounded like a good deal considering I was paying a lot more before. I advised the lady that I don't want pro rata charges so she needs to start it on the correct date. I was assured that would happen and advised that I would not need to do anything further. In the same call I told the lady that I need her to insure that there is a cap as I know out of bundle data charges are real heavy on the pocket. She again assured me that I would need to buy data or airtime to use the SIM if I run out. I was also told that this is a data only contract. The end of the month I check my bank statement and found that they took over r300 for this so called SIM deal. I call them up and ask why that has happened. Apparently the girl who I thought was so lovely signed me up for a phone that I never received and then they charged me for it. So they then admit their mistake because of a delivery number or whatever. Again I'm understanding. To this day I have still not received my refund.

Today I call them up and asked where the refund is and I get told there is a credit on my account and I didn't inform them I want a refund, I told the lady that I did and asked her to arrange the refund for me. After the call about my refund I get a message saying I'm going over my credit limit. Where and how is that possible when my data refresh on the 14th and today is the 11th? I don't mind running out of data even though I have WiFi at home and at work I know I use a lot of data and don't mind buying but I don't want to pay for something I'm not aware of. I checked my unbilled amount and find there is over r400 that I will be charged. My question is what is that for when I was advised that would not happen at all. I'm really very upset about this because I wanted to cancel due to financial issues and now they are charging me even more for something that is not my fault. They didn't even apologize.