Field Security Maintenance lets you control access to who can change fields. You should be able to set TermsCode to read only, which would force the user to accept whatever terms are set on the customer.
This. The Terms.TermsCode is a primary key for the master terms record. You don’t want to touch that. You just want to prevent the order entry peeps from modifying the OrderHed.TermsCode on their individual sales orders.