<?php
function remove_order_meta_box() {
// remove_meta_box( 'woocommerce-order-downloads', 'shop_order', 'normal' ); // Download at shop order page
remove_meta_box( 'postcustom', 'shop_order', 'normal' ); // Custom field at shop order page
}
add_action( 'add_meta_boxes' , 'remove_order_meta_box', 50 );