Cancellation of ADT Security membership

I have cancelled my ADT account in March 2017. My account was paid by a debit order. Once cancelled I have put a stop payment on my debit order aswell. Unfortunately they didn't cancel the debit order on their side and was returned by the Bank. They only sent me a notice stating my debit order was returned. I have tried several times to get hold of their accounts department on the number stated on the message ***) but no avail. Eventually I phoned their call centre and get hold of Katlego Moroko who mentioned I have to send them an email to cancel my membership. This was never mentioned to me. In July have sent him an email mentioning that being a pensioner I cannot afford to pay the overdue amount. That is the reason I have cancelled my membership in the beginning of the year. He didn't came back, the debit order was not presented again and I accepted everything is sorted. Then, all of a sudden in March 2019 I received an request from Norman Bissett & Ass. to pay an amount overdue. Now I received a message from them stating they have issued a letter of demand on my Std Bank account. Why? Five years later? What did I do wrong? I have cancelled my membership in time. I am very disappointed. Thank you