Help Block "Deliver from Location:" for user login

Dear joe and others,
Thanks for all your help. In cash Sale Invoice or (Sales Quotation Entry, Sales Order Entry, Direct Delivery ,Direct Invoice ), I want to block "Deliver from Location:" for user I set up. How can I do?
I create 3 users, I assign to 3 Locations. When user login, "Deliver from Location:" is assigned for him. But I don't want them edit or change.
Can you help me. I don't want salesman of this Location sell products from other location.
Thanks so much!