Forms Authentication - Signing out a user programatically

I'm wondering how to programmatically log a user out, using Forms based authentication.

Online users:

• ADMIN


• User1


• User2


• User3

If I'm the admin, how can I boot User2 out of the system? FormsAuthentication.Signout() would sign out/delete the cookie from the current user (Admin), which would be quite pointless.

Otherwise, I could put a flag in the database - on the next visit, an action filter would force them to the sign out page and make them log out. But that seems like a hack.