Overcharged again and again

MTN has been overcharging me for the last 2 months. I just discovered that I will again be overcharged this month. It is impossible to get hold of their customer services, head office does not answer their phone, the store I deal with refuses to help me and online support tickets are ignored. After my last bad review on this site Lungi responded that my account is credited but if I want a refund I need to send them bank details. I gave them my bank details on a support ticket online, as nobody will give you any contact details, but this too was ignored and I will be overcharged again. I just don't know what to do anymore, as they just don't seem to care about their customers and I just have to pay whatever they decide I should pay monthly. My contract is a top-up at a fixed monthly amount. Surely this is breach of contract!