Wrongly being threatened and not responding to emails

MetroFibre wrongly handed me over to Hann Collectors. I have forwarded the credit Invoice from MetroFibre to the email address that Hann Collectors provided, but i receive no feedback. I have made a lot of calls to them and spoke to a lot of people, but still there is no progress. I receive messages daily that i should make payment, but in reality i have no outstanding balance. I am now on my final warning before they take further action, and i am afraid of how this will effect my credit score. They don’t care about how peoples life's are affected. If they read there emails and do further follow ups, they will see that they are wrongly harassing me.