Data bills over the alloted 2 gig limit

I have a problem with my mobile service provider MTN. <br> <br> For the past few months they have been charging me exorbitant amounts of money for my data line. I have requested them in writing on two different occasions to limit my data use to the 2gigs that I signed a contract for. I tried to contact my credit controller via her telephone number to request it verbally but to no avail.<br> <br> I use my MTN data sim for approximately 15 days in a month and according to the MTN service *141#. I had 4 gigs of data available when I inserted my sim into my device. When I took it out, I still had approximately 3.6 gigs still available on the 6th of august. But MTN billing says that I used up the entire 4 gigs allotted to me PLUS another R6422.84 ex vat odd worth of data on top of that after I had already requested them twice to limit the data to the 2 gigs that I signed for. If this is the case and the usage has gone over the allotted 2 gigs that I signed a contract for, why did they allow it to happen and why didn't they notify me that I have gone over my allotted data. <br> <br> I know I didn't go over my data and I refuse to pay for this account as it is their fault and not mine<br> <br> alex hockly<br>