ADT harassment

In 2017 Nov I installed the ADT alarm then in Feb 2021 I decided to cancel it. I was given run arounds and they said that they'll reduce the premium but we could not agree with the price. I then submitted the cancellation letter in April 2021. They later called me and claimed that I owe them for May premium but after proving to them the cancellation was done the previous month they'll say they sort it but to my surprise I still receive sms that I owe them.