excessive billing

I've been billed excessively for my account ,I've got the 2GB option and once it depletes ,I'm suppose to purchase data or get authorisation to go above the limit.But to my surprise in December last year I got a shock of my life when I got billed over R400 where I usually payed R89 and when I quiried it they can't come up with reasons why it was that high,and I didn't even use the internet that much and then I insisted that they change it so that I can only go R50 above my limit and so my shock it happened again now in January!This is nonsense!!! How on earth all of a sudden I get bills of over R400 when my usage was as per usual and maybe even less.I want to end this contract,this is a ****! Ripping people off of their money,making an easy buck.