if :system.record_status  = 'INSERT' then
		declare
			vcour_code               ACCOUNTS.cour_code%type := NULL;
		  vcourier_account_no      ACCOUNTS.courier_account_no%type := NULL;
		  canvas_name varchar2(30);
		begin
			select cour_code,courier_account_no
			into vcour_code,vcourier_account_no
			from accounts acct
			where acct.no = :orde.acco_no;
			if 	vcour_code is not null then
				  hide_view('CG$TAB_1');
					:cg$ctrl.cg$stack_control := 'CG$TAB_2';
		 			show_view('CG$TAB_2');
		 			soft_messages('I',FALSE,'Customer''s courier preference is '||vcour_code|| ', Account: '||nvl(vcourier_account_no,'N/A'));
	
			else 
			  		hide_view('CG$TAB_2');
			  		:cg$ctrl.cg$stack_control := 'CG$TAB_1';
						show_view('CG$TAB_1');
			end if;	
			if :orde.shipping_method = 'R'
				then set_item_property('ORDE.COUR_CODE', NAVIGABLE, PROPERTY_TRUE);
						 set_item_property('ORDE.COUR_CODE', INSERT_ALLOWED, PROPERTY_TRUE);
						 SET_ITEM_PROPERTY('ORDE.COUR_CODE', UPDATE_ALLOWED, PROPERTY_FALSE);
						 SET_ITEM_PROPERTY('ORDE.COUR_CODE', REQUIRED, PROPERTY_FALSE);
						 
				end if; 
		end;
end if;		
As you had adviced i tried the above SET_ITEM_PROPERTY built in. However still not working. Still i could see the 'LIST OF VALUES' when the CURSOR is at the field COURIER CODE.
Have i made any mistakes?
Regards
Praveen.R

-- key-listval trigger
if certain_condition_is_met then
   message('Field is protected against update');
   raise form_trigger_failure;
end if;