You can manage Notifications in the Xfinity Home App. After opening the app, go to the Automation tab at the bottom. Here you will see a list of Rules.
You will likely have one for "Receive notifications when the system arms or disarms." You can tap on that Rule to edit the parameters (i.e. maybe you only want to receive notifications on certain days of the week, or at certain times of the day), or click the switch to turn it off entirely.
I hope that helps!