Blacklisted

Approximately 7 years ago I had a 2 year contract. About 5 ago I was told by legal letter I owe an additional amount, not sure of the amount. This was handed over to a recovery agency which I had to pay. After going to see them and resolving issue via telephone (Kuils River Branch) and paying the said amount I thought it a thing of the past. Only to be contacted again by Siya recoveries saying my account is in arrears. This is ridiculous! After years I get contacted again. Contract expired in 2011. I have had enough. What legal action can be taken against them as I have never missed or had a bad record with account payments. Calling the call centre is useless as they transfer me from one person to another.