How can I avoid unauthorised loggins

Hi
me with C# asp.net
In my web application I want to show my admin page only to users who had logged in, for that I had made a login form for enter username and password and if the username and password is correct I set as session["authen"]="Yes" 
then in the pageload event of the admin pages I checked whether  session["authen"]="Yes" or else redirected to login page and also put a signout button to make session["authen"]="No"
but the problem is that when a user logged in and visited the admin pages and signed out if we take the browser history and select a page its logging in
how can I avoid this 
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