Angry customer

Hi, I'm writing this review being very upset with the company. I was using their service for the past four years and I sold my house last year and I called them to ask how do I cancel the alarm service as I'm moving out and they told me to write an email about it and they would cancel it and the guy who was helping me also told me that I called at the right time for canceling as if I called later they were going to charge me for December 2020 but I'm safe and it was canceled. To my surprise they deducted the amount in December and doubled it for that matter and I reversed it as I thought this can't be I canceled and I told them I'm moving out now they are sending me smses to say I'm owing them for December and im going to be blacklisted if I don't pay and I'm trying to explain to the guy what happened and he says there's nothing he can do I must pay and mind you my month ends on the 15th and I had already paid for November 15th until December 15th and I canceled before the 15th of December 2020 of which they told me it was the right time for me to cancel and didn't say anything about paying for December and January 2021of which I had already moved out