To make .aspx page only editable by one person at a time.

Hi,
  I have a page with some fields,authenticated users can update the data. I need to make this page only editable by one person at a time.
  So if a user enters into the page,i need to lock it. For this i have added one column in DB(flag),in 'Page_Load' event i set that flag as '0'.
  Now in 'Page_unload' event i need to unlock that.
  The problem is 'Page_unload' also fires in load event. So i choose window.onbeforeunload and window.unload. In this event i can return the cofirm msg only.
  I can't call server side script[ie)code behind function]. Any idea.
  I'm using asp.net and c# as frontend and SQL as backend. Thanks in advance!!!

Regards,
Narayanaselvi.G