Post as frontpage – avoid duplicate content

Your best way would be to redirect to the theme’s 404 page in the event that the page being loaded matches the slug that you are referring to. Try this:

function check_undesirable_page(){
    global $post;
    if(is_page() && ($post->post_name=="YOUR-SLUG-HERE")){
        global $wp_query;
        $wp_query->set_404();
        status_header(404);
    }
}
add_action( 'wp', 'check_undesirable_page' );

EDIT: Also a good idea to incorporate the is_page() condition (added above) to avoid undesirable consequences of the page sharing its slug with other items (in case it’s a common word)