Programmatically Logout A "specific" User?

Is it possible in Asp.NET MVC to proframmatically logout a user? I am aware that you can use:

FormsService.SignOut();

But this refers to the context of the webpage making the request. I'm trying to prevent a user logging in twice. So if I call:

MembershipUser membershipUser = Membership.GetUser(userName);
if (membershipUser.IsOnline == true)
{
// log this user out so we can log them in again
FormsService.SignOut();
}

Calling FormsService.SignOut(); has no bearing on the context of the user say, with another webbrowser who is logged in already?