How to Make Field Named Shipment on Edit Shipment Page Non-Editable?

Business' system is set up in such a way that once we do the Create Pick Wave for an SO under the corresponding release rule, the line status (as seen on Manage Shipment Lines) changes from Ready to Release to Released to Warehouse. It is at this stage that the shipment number is auto-generated and assigned to the shipment line pertaining to that particular SO. When we click on the shipment number, the Edit Shipment page opens, where the field named Shipment is editable. When we do the Confirm Pick Slips (the Confirm and Go to Ship Confirm option), the