Keep on making me pay more, after I payed one

I have payed one contract in cash and was still charged for it for 3 months, had to battle for almost 5 months to get that sorted out and then I did not receive a refund... After a long Wil they only debit my account with R 100 rand and then phone me and tell me a owe them R 3000 all of a sudden, if have never had such a bad experience in my life when it comes to customers service and refuse to pay anything else until this matter is sorted out... and do not think you are going to let one of thsose *********** people phone me, you people are steeling my money and are unprofessional