First, Make P6_REGION_ID

that will contain all Region available for your page

use this code as a source

select PLUG_NAME, ID from APEX_200200.wwv_flow_page_plugs where FLOW_ID =:P6_FLOW_ID and PAGE_ID =:P6_STEP_ID

and make a process to update Region read-only

BEGIN

UPDATE APEX_200200.wwv_flow_page_plugs

SET PLUG_READ_ONLY_WHEN_TYPE = 'EXISTS' ,

PLUG_READ_ONLY_WHEN = 'SELECT 1 from dual where 1 =2 '

WHERE FLOW_ID = :P6_FLOW_ID

and PAGE_ID = :P6_STEP_ID

and ID = :P6_REGION_ID ; end;