write this code in logout link button
Session.Abandon();
Response.Write("<script>window.top.navigate('../login.aspx')</script>");
the above script disables the back button of the internet explorer
or else you can write like this
in each page in Page_Load event
protected void Page_Load(object sender, EventArgs e)
{
if (Session.Count > 0)
{
}
else
Response.Redirect("Login_page.aspx");
}
Hope this will help you