want to sue

Approximately 8 years ago I prepaid a 2 year contract. End of 3 years later I get told I owe an additional R1800. After going to see them and resolving issue via hello peter I thought it a thing of the past. Only to be contacted again today! To be blacklisted. This is rediculous! Does the national credit act mean nothing to them. I have had enough. Sue for distress