paste the following code on your Login.aspx. under <Head> section
<script language="JavaScript">
javascript:window.history.forward(1);
</script>
Note: above code will works only in IE
but in my view. You need to think again for your application logic so that it doesn't matter if the user presses the back button.
On you main menu screen (the page after login screen) load event, check if the
Session["UserName"] == null if yes then show error message "You have logged out, please login again."
when you click on logOut call
Session.Abandon(); which will clear all your session variables. and when you click on back button it will through you message "You have logged out, please login again"
it Smoothen your flow and browser independent.