I am looking at how best to prevent a single user account logging on multiple times in a webforms application. I know that MembershipUser.IsOnline exists, but I've read a few forum and blog entries suggesting that this can be unreliable, particularly in scenarios where a user closes a browser (without logging out) and attempts to logon with a different machine or browser.

I looked at implementing a last past the post type system; when a user logs on older users are simply kicked off. It seems that FormsAuthentication.Signout() only works for the current user.

Am I missing a trick, is there a better way to prevent the same username logging on from multiple different locations?