Collecting on prescribed accounts

Marang Credit Solutions is now harassing me, they are collecting on prescribed debts , i have explained , and sent emails over and over again but they are still calling me

This morning i was contacted by a Thulani Shongwe who did not even want to listen to me, he kept telling me that they will keep calling me , wich i don't understand because the account is over 10 years now, the credit provider ( Thruworths) even approved me for a new account last year. What they are doing ( collecting on old accounts) is illegal