DISPUTE ON HUGE BILL RECEIVED

In Oct 2014 I upgraded to a iPhone 6+. I asked the sales person at MTN to put a EXCEED LIMIT on my account that I can't exceed R100-00 per month. For 17 months my account has not exceeded R1 200-00. For 17 x months I added data on my contract and husbands data bundle if we ran out of data - without ever getting a nasty bill. We could never go on internet if the data was finished. End of Feb my husbands bank account was deducted over R13 000. Now I am trying to dispute the big amount that was deducted. DON'T AGREE WITH THE AMOUNT BECAUSE I WAS ALWAYS UNDER THE IMPRESSION MY ACCOUNT LIMIT WAS SET AND I BOUGHT DATA ALMOST MONTHLY. SO WHY ONLY 1 X MONTH THAT THE ACCOUNT WENT WILD. MTN said I must dispute it with the lady at MTN so that we can come to an agreement. Funny that she is not working there anymore. After lots of phone calls and already 1 x failed dispute I logged another dispute this afternoon. It was simply said that whether you are a client at MTN for 1 year or 20 years. It was said - If MTN feel you used that data you will have to pay for it. Well now I basically paid because the sales people can't do their job. WELL DONE MTN - IS THIS THE WAY CLIENTS ARE TREATED???