MTN overcharges on Top Up Contract

I have a Top Up Contract with MTN which costs R499 per month. However every month they charge me for extra data that I did not ask for!!! I phoned the customer care number, they said I must go to the branch. I went to the branch, they say the customer care people should have helped me over the phone. Then they reverse one month's charges - just to charge me again for data the next month that I have not asked for. Since I got this contract upgrade in January I have only once paid R499, since every other month I get charged extra. Do I now seriously have to go to the branch every month to get them to fix their own mistakes? I have had it with MTN.