only one user logged in

Hello! I have a program with which the administrator can setup some system. I need only one administrator to be logged in at a time. If, on successful login, I set the sid in the database and erase it on logout I have problems when the logged in administrator closes the window or anything else that does not erase the stored sid. And if he wants to login again the programm will say that there is another administrator logged in. I tried to put php to clean the session_id after, let's say, 40 mins with a probability of 100%... but if the administrator closes the window accidentaly he won't be able to login for the next 40 minutes. That is not very nice. Any ideeas? plz help Mihai