Redirect custom post type single page to not found page
Here are the steps to redirect your custom post type single pages to not found page. Add the below code in the end of your theme functions.php
add_action( 'template_redirect', 'redirect_post_type_single' );
function redirect_post_type_single(){
if ( ! is_singular( 'post_type_here' ) )
return;
wp_redirect('/404');
exit;
}