Account should have being closed by service 2014

Account no:***2***4, which was paid April 2014, and with credit balance by that time. And the account now have R1600.00 outstanding amount which we don't know where this amount is coming from. we requested the statements to show the items purchased after the date of payment made in April 14. We tried to get the attention from the service provider but with no success. Now the account has being handed over debt collectors by the name of vericredoline which is charging excessive amounts. As the result we are charged the amount which we never collect any items from store.