Matrix will not cancel my subscription

I have had my car tracking with Matrix for 5 years but recently sold my car. Getting this service cancelled has been a nightmare. After being pushed from pillar to post I eventually spoke to a manager who assured me it would be cancelled only to receive an email 3 days later saying there has been an issue and they can’t cancel my account. I have now been paying for a tracker for a car I no longer own for 2 weeks and I’m no closer to getting it sorted.