No assistance from the accounts department and cancellation not processed

For a number of years we gave money to ADT - not for any personal services but for them to look after our area - a charity amount as such. In July 2020 after our business took a huge nock we decided to cancel this amount. I cancelled via email with the correct correspondation as they requested. In December 2020 we once again received a statement showing that we owe from July until December 2020. I once again cancelled and agreed to pay the amount outstanding as I really did not feel up to a fight. I once again now received a statement for 3 months. After numerous phone calls holding on for more than 30 minutes I am only being helped now - saying that it is for the notice period. They have a blatant disregard for my cancellation in July 2020 ignoring it and still extending my notice period into February 2021. I will never do business with them again. Imagine if I was actually paying for a service that they offered. Once again I am being penalized because in July 2020 someone else did not do their job.