Why is it difficult for MIx Telemetics to cancel my account.

I have sold my car on 09 September 2021 and notified Mix Telematics. I was then contacted by an agent who assured that the account will be closed. The account was not closed as it tried to withdraw money from my bank account. I reversed and blocked it from withdrawing money. I wrote 2x emails requesting the cancellation, I received responses with reference numbers that the requests will be attended to. I now see that I have missed 2 payments on the credit bureau's site. Can Mix Telematics up its antics and stop playing games with our monies and credit scores, and cancel the account? I expect it is just lying low waiting for the right time to report me to their Debt Collection Department. I NEED THIS ACCOUNT CANCELLED YESTERYEAR.