If you don't use a device but it still appears in your devices list, here's how to remove it:
Go to account.microsoft.com/devices, sign in, and find the device you want to remove.
Select Show details to see info for that device.
Under your device's name, select More actions > Remove.
Review your device details, select the check box, I'm ready to remove this device, then select Remove.
If you want to remove a device for a child in your family, have your child sign into account.microsoft.com/devices, then follow the same instructions.