How to logout another user in 10.8?
As an administrator:

sudo su - user_to_be_logged_out
kill -9 -1


That will log out any user by ending all of the user processes. Use care to not issue the kill -9 -1 as root else some work would be expected to be lost as the system would immediately self-destroy all processes.

There are of course one line commands, but I like having a pause to look and make sure I switched to the correct user before issuing a powerful command like kill -9 -1 and I also avoid at all costs having sudo appear before that kill since I don't even want to kill all processes as root.