Issue in Sales Order.

I have created a sales order ,outbound delievery, Post Good Issue and then Invoice with 100 quantity of FG material, that means that the sales cycle is complete and the document flow is showing that the sales order is complete.

what i want is if slaes order is complete system should not allow to modify or add any information in sales order but when i opened the same sales order in change mode and add new item line with another material and saved the sales order system has not showed any error or waring that the sales order is already completed, now the document flow is showing that the sales order is incomplete.

please help me out that how can i cater this situation, that once sales order is completed system should not allow to add any item line or modify any information in the sales order.