NRB Solutions- do better!

I have been consistently called by this company for fees incurred on a Nedbank account which was closed (obviously incorrectly on Nedbank' side) more than a decade ago. I have explained that the the account was officially closed and the error lies on the bank's side so they need to correct it. Instead NRB just keeps calling. Love how they try to squeeze money out of people that are legitimately on the right side here. This feels like harassment, how long do I get held liable for a banking issue?