Constant harassment and threats, do not use Nedbank

For the past 5 years Nedbank has been harassing and threatening us (including WhatsApp calls while abroad) regarding a supposed arrears in our home loan. We have explained countless times, we have sent countless statements to prove there is no arrears, we have been to 3 different bank managers in the branches and yet the harassment and threats continue. How can an institution like this not fix a admin error and treat their customers of decades in such a manner. I would not recommend using Nedbank at all.