ADT desperation

We have been using ADT for a number of years. As we are moving on we decided to cancel the service and did so in writing on the 1st February 2023. We paid that months installment and then I had to return another installment in March 2023 as the notice period had been served (1 month in advance). They will not accept that the service has been cancelled and I now have Attorneys trying to leverage money out of me. This is just pure harassment - please don't use this Company - I have already barred them on our home owners association