How to turn off searching from URL and go straight to 404 page

Try

function no_attachment_page() {
  global $wp_query;
  if ( $wp_query->is_attachment()  ) {
    $wp_query->set_404();
    status_header(404);
    nocache_headers();
  }
}

add_action( 'template_redirect', 'no_attachment_page', 1 );

This should result in a 404 everytime you try to access to an attachment page.