Harassment and poor management

I've been harassed by altech netstar to pay an outstanding balance of which I don't know the owner of the car.In 2016 I cancelled tha account I had with them and paid up in full tje amount owed ,and they even sent me a paid up letter. Last year 2017 I started receiving call,emails n smss that I have n amount that is due n need to be paid.I contacted them via email n ask the registration number of the car.they sent me the car registration number and I found out that I don't know that car.I even told them to trace the owner n let her/him pay.Then the matter was quiet for some time,it rose again in 2018. I feel I've been harassed by them.How can i pay for the car that is not mine.I only have 1 car,never owned 2 My name at credit bereau will be tarnished because they are saying I owe them.Please sort this mess out and trace the rightful owner of the second car.