Cancelled ADT account

I wish this company will listen. They keep on bothering me with an old ADT account. I have cancelled my membership with ADT Security Co. My account was paid by an debit order and was paid fully up to date. Once I have cancelled my membership with them, I cancelled the debit order aswell. Unfortunately ADT did not cancel the debit order on their side. Every month they send me message regarding my unpaid debit order (stopped by me). I could never get hold of them until one day when I got hold of Katlego Boroko who mentioned they need my cancellation in writing. Nobody told me that and I couldn't find that in my contract either. I have mentioned to him I, being a pensioner, cannot afford to pay the overdue amount should they only cancel my debit order at that stage. The didn't contact me again and the debit order was not presented again. I accepted everything is sorted out. Then, almost 3 years later, I received the first call regarding ADT. I mentioned this to them and now, almost 5 years later, they still threatening me, this time with a letter of demand. Why? I have cancelled by membership and ADT didn't do it on their side. This wouldn't go that far should the staff answered their phones in the first place.