Deleting Entry From LOGGEDINUSER TABLE

I need to delete an entry from the LOGGEDINUSER  TABLE whenever the  user logs out, using the dedicated log out button i can accomplish this easily, My question is when the user closes the browser without clicking log out button how can i delete the entry from the  LOGGEDINUSER  TABLE ???. I need to do this in chat application where real information about currently logged in users is paramount.  ...