Force user logout?

[Alan Dayley]

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Let's say a user logged into a computer and set the screen saver
password (My son did this against house rules to keep his siblings off
the computer).  This user is no longer available (He is now in bed).  I
don't know his password but I know the root password for the machine (Of
course!).  I want to force the user to logout so the machine can cleanly
shutdown.

Google provides many answers and I haven't found the clear one yet.
Logged in as root on another console, how do I force a clean logout on
the (now sleeping) user?

Alan
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