Session does not expire? why?

[eluser]mindprojects[/eluser]
All right this is the solution:

Session won't expire on browser exit,session will espire only after $config['sess_expiration'] time is expired from last login.
The value of 0 means no expiration!.

Now this is the problem:
i have a site and people access the web app from public places.So i need to avoid that if a user is going to close his browser session data keep living in session for the next user.
The solution,i think,is deleting the session on browser exit,isn't it?
How to do that?