MTN overcharging on data contract

I have a 2 gig data ****le contract with MTN. When the contract was set up I asked the consultant to cap it because I am weary of the out of bundle tariffs. My contract is for R99 per month. When the June 2015 invoice arrived I was shocked to see it was R1515! After holding for the call centre and not getting through I sent customer service an email on 25 June. They only rep**** on 14 July, and it was not even a proper response. I then asked them on 14 July to cap the monthly data with immediate effect. The July invoice was R105. Then came August - R3841!! I could not believe it. I was constantly watching the outstanding balance. There must be an error somewhere. This is becoming rediculous. I fear they are going to take my money via debit order and I can do nothing about it. MTN please stop ripping me off and give my money back. Surely a credit is in order. I asked for a capped account in Jan 2014 already and again in July 2015. Your customer support is appalling and your customers are the ones paying the price for it.