UNDUE CHARGES ON ACCOUNT

I have been given a credit limit of R3000 by rcs/coricraft. On the 13th August they sent a statement saying i need to pay R2193.15 to bring account up to date. On the 26th August I settled the account with a payment of R2194. On the 12th september/ 12th October/ 12th November the they have been charging interest plus R68 per month of service fees on a zero balance, and now I owe them!!!???. They do not want to explain why and how these charges were calculated and on what basis as the account was settled on the 26th August. Can someone from the RCS/Coricraft group contact me to explain these charges as to me this is not right. I want to close my account for good. This is really bad service.