Unfair treatmen by Game Stores

i settled my account in September last year as per what was on their system on that particular day, only to learn later that there was an amount of R25.00, which was not showing on their system. One of their agents called few months after that to tell me i am still indebted to them. and by that time the account has already accumulated charges like interest, collection fees, customer protection insurances, collection letter fees, and this amount is increasing every month of which i aske my self which debt are they protecting as i have settled what i owed. I have been threatened that i will be blacklisted if i don't pay what is currently on their system which have been escalated to R151.00 as of today, i feel bul**** by them and have taken a decision that i will not pay anything towards that debt. I feel i am unfairly treated