/**
 * Don't prepend default WordPress slug for CPT
 */
add_filter( 'register_post_type_args', 'custom_change_cpt_slugs', 10, 2 );
function custom_change_cpt_slugs( $args, $post_type ) {

  switch ( $post_type ) {
    case 'arts_portfolio_item':
      $args['rewrite']['with_front'] = false;
      break;
    case 'arts_service':
      $args['rewrite']['with_front'] = false;
      break;
    case 'arts_album':
      $args['rewrite']['with_front'] = false;
      break;
  }

  return $args;
}