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I think Microsoft Windows can be setup to turn off Wi-fi when it is in sleep mode, but I'm not sure if it is possible with Ethernet.

No there isn't, I've tried as I've had a few paranoid people asking.

I think setting a software firewall to disable all internet and network traffic may work. I wonder if there are software firewalls which can turn off a network card after a user have not used their computer for a few minutes.

firewall can get messy if not set right, it could block basic things and irritate people. I try not to mess with it.

Network card makers can make a switch which turn off the Ethernet when you plan on not using it. Turning off the network card may save a little bit of electricity.

Using the Windows device manager or Networking control panel program to disable the network card would work as well.

I think some old machines did come with switches you can hit, with towers you could take off the wifi antennas or hit a button on a laptop to kill the wireless.

I had a old laptop with a physical switch for turning off the wi-fi.