Is there a way to prevent multiple sessions for the same user from starting?

i.e. from screen saver lock, if you hit switch user, you can either activate an existing session, or create a new one, even if that new one duplicates an existing sessions. It's that last functionality I'd like to disable (duplicating an existing session). Basically, my kids have a login, and they keep creating new instances of their session, instead of reusing an existing one.

So, is there any way to prevent duplicate sessions from the same user?

Thanks,
Pete

see viewtopic.php?f=22&t=96580&hilit=sessions