Shutting Down A Browser

i'm trying to keep users from being able to login to a site twice at the same time. everytime a user does a login, i stick their userID into an application scoped array. if they try to login again, i bounce them to an error page. i use the session_onEnd sub within global.asa to remove their userID from the array at the end of their session, and this seems to work fine.

however, if i just shut down the browser, the sub_onEnd doesn't seem to get called, so i'm unable to remove their userID from the application array. any ideas on how to force this sub to fire, or is there another way around this?