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You can use the Terminal to shut down any Mac on your network that you have administrator access to.  You can find the Terminal in the Utilities folder in your Applications directory. You can do this from Windows by using an SSH program such as PuTTY to connect to the Mac via the command line. See Use SSH on Windows for details on using PuTTY. Once you're connected via SSH, you can use the same following commands. ssh username@ipaddress. Replace username with your username for the remote computer. Replace ipaddress with the remote computer's IP address. See Find Your IP Address on a Mac for details on finding a Mac's IP address. After entering the command on the previous step, you'll be prompted to enter the password for that user account. sudo /sbin/shutdown now and press ⏎ Return. This will immediately shut down the Mac computer remotely, and your SSH connection with the computer will be severed. If you'd rather restart the computer, add -r after shutdown.

Summary:
Open the Terminal on another Mac on your network. Type . Enter your user password for the remote Mac when prompted. Type .