Intercepting posts in WP

function.php

<?php

function my_pre_get_posts( $query ) {

	if ( ! is_user_logged_in() ) {
		$query->set( 'post_type', '404' ); // Do something that you'll never get results
		return;
	}
}
add_action( 'pre_get_posts', 'my_pre_get_posts' );