Impossible to cancel subscription

I have sold my vehicle fitted with a Tracker device months ago. I have been trying to cancel my subscription for months. If you try to do it online and you fill in all your details and select "cancel subscription" it gives an error "Oops, something went wrong". I have tried it multiple times over months. If you phone them, I never get through to an operator. But every month the debit order goes through. No service, I do not even own the vehicle any more, but still they take my money.