How to Disable delete orders

If you want to remove the order suppression, the best you can do is remove the DELETE access rights on the jos_hikashop_order table in phpmyadmin. That way, you will be sure that nothing can delete the data in there. That can be done in phpmyadmin providing that you have enough access rights to do it.

Alternatively, you can empty the delete function from the file administrator/components/com_hikashop/classes/order.php so that you leave only that code:
function delete(&$elements){
}

However, the second option will be overwritten when you update hikashop so I would recommend option no. 1