Poor service and **** from Cartrack

I cancelled my Cartrack account because I sold my car, Cartrack advised that since I'll be cancelling a contract I must pay a settlement amount. I paid the full settlement amount that Cartrack gave me and now Cartrack agents keep calling me saying I still owe them and threatening to blacklist me. This is very much unfair as I have paid the settlement amount in full which I was advised by them. They refuse to close the account and they continue to harras me calling me threatening to blacklist my name . Please kindly assist me. My contact number is ***