Question about logins and locking

Hey everyone, I have a question about logins. Let's say that I want to allow each user account to login only once at a time. I would then need some kind of locking mechanism to make sure that the same user can't login again somewhere else until first logging off. What's a good way to achieve this? I want to be able to handle situations in which the user closes their browser without first logging off, where I would want to count that as a logout. Perhaps I could do some kind of periodic polling in Javascript, combined with a query to the database that sets a value when the user logs in and when the user logs out? I'm just looking for some conceptual ideas. Thanks everyone! James