remove session objects

I have a web based application. It uses session objects. We have a logOff buttion in the application. When the user clicks the logoff buttion the session objects are destroyed and logged off message is displayed.
Any thoughts on how to destroy session objects if the user closes the browser without using logoff buttion? I tried onunload event but this is a problem as his objects gets destroyed if he clicks refresh on the browser.

Thanks in advance.

what sessions? what is the code? theoretically all sessions are terminated when the browser is closed. but what code are you using?