MTN Charging more than my credit limit

MTN has billed me with R7600 when I have a credit limit of R2000. I want my money back! No one can give me a reason for this. I have logged many complaints via phone and mail and nothing happens. I aways receive feedback saying 'we are terribly sorry for the inconvenience, it was not our intention'. They seem to make a habbit of it and i am sick of it! The sevice is terrible! The people in the MTN stores just call the customer care, the same as i do so, they are useless! The customer care people send you to all the other departments till you get back to the first one and the cycle continues! No one can/will help me! As a service provider there is little attention given to CUSTOMER SERVICE! Ive been with MTN for about 13years. When this is resolved (if such a thing is even possible with MTN) i doubt i will ever go on with making use of MTN or recomend it to anyone!