I developed a code hack that will block simultaneous users and limit the login session to a given amount of time.
You could take my code and modify it to create a timer that would require the user to log-in again or just ask the verification question.
There is also a plugin available to block simultaneous users, but I haven't looked at it yet.
Read the posts in this forum http://moodle.org/mod/forum/discuss.php?d=34457#p674481
The link to my code is in that forum.