V$session : Session not Closed

I connect to Oracle DB thru JDBC. I used CLIENT_INFO field in V$SESSION to store my intranet Application's User Name along with Username.
I controlled the user access thru checking that field. I assumed even if user goes out of that page, session will be closed automatically. But one of client went to other page by mistaken. He restarted the IE as well his own machine. But still the session is not closed. I can see the session in V$session.
Any idea whether such problems are preventable. i cannot even close that session from my machine. How to close that session.
Thank you in advance..Note that you probably want to address JDBC questions to the JDBC forum rather than the ODBC forum-- different protocols, different products.
Justin