MTN refuse to give me my refund of over R4000

I had a contract with MTN for a phone, when I went to pay off the remaining amount left on my contract MTN over charged me by over R4000 and when I try to get my refund MTN also cancel my claim and claim that it has been dealt with. Is going to a year where every time I try to get my money the say the claim has been dealt with and closed. As big as MTN is as a company it is not reliable and will never advised anyone to buy at MTN. They are many phone companies that get such issues in within a month people get their money but MTN has proven time and time again that they are not a trusted company and I will never again buy anything at MTN rather go to Vodacom, Telkom and many more others before ever going back to MTN.