Double debits but no Refund

I am with MTN FTTH and last year November I was migrated to Supersonic (by MTN). Since then I have been getting debited by both MTN and Superonic. As a result MTN/Supersonic now owe me money, which they have acknowledged. I have been trying to get a refund or my account credited for the amount owed to me for over 5 months now but still nothing. We are at the stage where Supersonic just ignores my requests and hopes I will go away. They are still holding onto my money and not giving me the refund owed to me. Anybody else having this problem?