limit concurrent logged sessions

What would be the best approach to limit the # of sessions that can be active at one time. Page access is restricted by session variables. In an extreme example, someone could give out their username and password to 100 people, and they would all be able to login at the same time. I want to limit to one person at a time. Any advice?

Obviously the login script will need to add timestamp or trigger something in a tracking database table... however, without the users logging off, how would I allow a new login once that browser is closed or that session is timed out.