Via Netflix, you can only remove 'download devices' individually, not actual devices, unless you remove them all using the "Sign out of all devices" option. But, if the device is on your home wifi network and your router allows you to block content, you can log into your router settings and restrict a particular device from using Netflix on your wifi. Of course, you can also log out of the Netflix account on the device if you have it available. Following the exact instructions as above, tap on the option to 'Sign Out' from the menu. If you no longer want that device logging back into your account, change the password for extra security.