Refusal to cancel membership for a car not tracked

A vehicle tracking device was installed on my 2013 Audi A4 more than 10 yrs ago. The car was involved in an accident and written off in September 2024. I informed CARTRACK in June 2025 that I no longer had the vehicle and would like to cancel membership and related debit order which they've unduly benefited from since Sep 2024 when I informed them the vehicle was in an accident. My request was ignored and I had to call them again in July at which point they tried to coherse me to agree to a verbal declaration which entails them charging me for another month. I refused and told them that they've not rendered any service for months and I cannot be expected to pay further for a cancellation and would be cancelling my debit order with the bank since they clearly refuse to do it on their side. Since cancelling DO with bank, they've harassed me with daily phone calls, SMS and emails of an overdue account, increasing amount owed every month even though I've cancelled membership. Please assist.