Warn users not to edit the post container

posts-page-warning.php

<?php
// Add to functions.php or place in mu-plugins

// Admin Notice on Posts Page
add_action( 'admin_notices', 'scl_postspage_print_notices' );

function scl_postspage_print_notices() {
	$screen = get_current_screen();
	if ( $screen->parent_base != 'edit' || $screen->base != 'post' || $_REQUEST['action'] != 'edit' )
		return;
	
	$posts_page = get_option( 'page_for_posts' );
	if ( !$posts_page )
		return;	
    
    if ( $_REQUEST['post'] == $posts_page )
        echo '<div class="error"><p>This page is a container for the most recent posts. It should always be empty, and you should never edit this page. To add a news item, go to <a href="post-new.php">Posts -- Add New</a>.<p></div>';
}