Bad Service from the Accounts Department.

I cancelled our contract November 2024 as our new car came fitted with a tracking device. I phoned asked for a dull and final settlement which they sent by email and I paid. I emailed the POP they tried to take the debit order so I stopped it with the bank. Every month I get a statement saying I owe money I received an email saying I hadn’t paid the December instalment which I paid and sent the POP and u received an email to say the account was closed. Not sure what their accounts dept don’t understand about full and final settlement. Today nearly six months later I am still getting statements with money due. I have repeatedly sent all the documentation in to them but it seems to make no difference you don’t even get a reply to your email or a phone call.