function wpf_only_allow_completed( $order_data, $order ) {

    // Only sync the order details if the order status is completed OR if the customer already has the tag MY TAG NAME

    if ( 'completed' == $order->get_status() || wpf_has_tag( 'MY TAG NAME', $order->get_user_id() ) ) {
        return $order_data;
    } else {
        return null;
    }

}

add_filter( 'wpf_woocommerce_customer_data', 'wpf_only_allow_completed', 10, 2 );