Good question @Helenev! You can exclude the Motion Detector from modes by either visiting the Motion Detector settings, or the Mode settings in the main Ring app Menu. Once in Modes settings, you’ll see a toggle for what sensors you would like to include.
If you are not wanting to use the Motion Detector at all, visit the advanced options of your Alarm system and “remove a zwave device” through there. This will remove the Motion Detector from your setup. I hope this helps! 