Overbilling for services not provided

After struggling to understand my high bills from MTN I decided to take my contract on my phone to pay as you go and to keep my data package on a 500 MB contract. I however buy my data-bundles from Afrihost and has a hard APN setting to only use the Afrihost data. This however only stopped MTN from over billing for the first month. Thereafter the billing escalates, I complain, MTN does not bill for the next month and then the billing escalates again. Over the past two months the billing went to over R2000 and R 4000 respectively. Each time I have to spend over an hour to get an agent on the phone that will make empty promises and a 5 working days window for feedback. I want to move back to contract with my voice bundle but have zero confidents in MTN's ability to correctly bill for their services. The whole billing issue seems to be full of *****... Consider this, since I've moved to pay-as-you-go my cell account has dropped to between R 100 to R 200 per month. Under contract I would have paid between R 1400 and over R 3000. Makes one think.