hi
i have similar problem to this topic:
www.joomlapolis.com/forum/153-professional-member-support/237999-how-to-realize-an-auto-logout-function
but your final solution to use flag system maybe not work. because they power off system directly via windows and not even close the browser. instead of they unflagge checkbox , i can say to him press logout button. am i right? but they not do that.
have you any solution other than CRON?
2-if i set session in joomla on php(not database) how can i delete their sessions? is it related to CB or it is joomla system?