Cancelled 18 months ago but still being invoiced

In Sept 2014 I cancelled the contract for armed reaction for 2 households & 1business as they kept reporting all in order at the office when the alarm went off yet windows were left open. Plus our home alarm was not sending the panic signal through. They attended the premises (charged R 800) & stated that the mag on our bedroom door was not connecting properly which is why they don't receive the panic signal. What a load of hogwash. We fixed the mag but (guess what) that they still didn't receive the panic signal when I pressed the panic alarm. Cancellation letters were sent followed with e-mails. Beg March 2015 & June 2015 & Sept 2015 they demanded money for the cottage or the Sherriff was coming out. I resent all e-mails each time. I also mentioned that my mom (who stayed in the cottage) had passed away. In Dec they phoned wanting to speak to my mom - really??? Then they asked for the death certificate. As a matter of principle I did not send it to them. In Jan they phoned again and I resent the 20 e-mails. Now I get a message from Debt Source as the account was handed over. Is ADT stupid?