I don't think there is a specific command to log out users, but what always works is killing their login shell
Using the "w" command you can see who is logged in + their terminal (e.g. pts/1 if logged in over ssh)
Then use "ps", it has a switch to show all processes and the terminal they are running on (I forget, will have to look).
Matching these two gives you the processes and their numbers (PIDs) that are running at that terminal. Then shoot them into oblivion with "kill -9 <pid>"