Legal action threats

I have been trying to sort out a problem with Netstar for over 6 months now. I started getting messages that I am in arrears, I emailed them to ask which account it was as I was not using Netstar for 2 years. The reply was that there is no account linked to my email address. The a debt collector started calling,I told him the reply from Netstar and he hung up. Then their customer service called me, when I told them about the email response, they hung up on two occasions. Then Netstar tried to register a debit mandate in my FNB account without my knowledge. When it was declined I got another message that there will be legal action against me, with a message that the received a panic alert from a vehicle that I sold long ago(over a year).