Prevent multiple users to log in at a given time

Dear Techies

I have an ASP application in which I want to prevent multiple users to log in at any
given time, using the same User Id.
I can do this by keeping a flag in database & checking it at user login time and re-setting
it at logout.
But what to do when session times out ?
This, I can handle using the session_OnStart() & session_Onend() events of Global.asa file.
But session_Onend() event is highly unreliable, hence I am again stuck.

Kindly guide me ASAP.

Regards & Thanks