Collecting of debt not owing

I no longer own a television. I did not report the loss to the South African police. I have done an affidavit to record this fact and repeatedly invited the SABC license inspectors to visit my premises. Despite all of this, Nudebt continues to try and collect a debt that they know is not owing. They continue to ask me to prove I don't have a television, but surely they need to prove I have one before trying to collect debt from me.