Kicking out other users

You must be superuser for both.

On Linux

Issue the simple w command:

[root@zenith ~]# w  
15:17:55 up 1 day,  3:35,  1 user,  load average: 0.06, 0.03, 0.01  
USER     TTY      FROM              LOGIN@   IDLE   JCPU   PCPU WHAT  
root     pts/0    dhcpw80ff96bc.dy 13:24    0.00s  0.04s  0.00s w  
tomc     pts/3    dhcpw70gghjbc.dy 13:24    0.00s  0.04s  0.00s w

Looks like the user tomc is logged into terminal 3. Kick the user
out of his session by:

pkill -9 -u tomc -t pts/3

On OS X

Find out the PID of the loginwindow process for a given user you want
to kick.

ps auxwww|grep loginwindow

In this case, we want to log out the user bennifer

bennifer    15015   0.0  0.2   281680   7188   ??  Ss   11:06AM   0:00.69 /System/Library/CoreServices/loginwindow.app/Contents/MacOS/loginwindow console  
root        15208   0.0  0.0    66152     88 s000  R+    1:30PM   0:00.00 grep loginwindow

Now kill the PID 15015

kill -9 15015