Grossly over charged on debit account without my permission

I have been with MTN for years. Have a top up 100 contract with them where I pay R279 a month. On the 3oth September 2017 I was billed R779. That is a difference of R500. I cannot afford such a large amount to be taken off just like that, now I am struggling to get my money back. The service has been horrific. An error on their side not mine yet they won't refund me the amount over billed. One consultant accused me of being confused. Never before have I as a paying customer been so badly treated.