.....
formname := NAME_IN(\'system.current_form\');
blockname := GET_FORM_PROPERY(formname, FIRST_BLOCK);

WHILE (blockname is not null) LOOP

  IF (GET_BLOCK_PROPERTY(blockname, BASE_TABLE) is not NULL) THEN
 
    SET_BLOCK_PROPERTY(blockname, INSERT_ALLOWED, PROPERTY_FALSE);
    SET_BLOCK_PROPERTY(blockname, UPDATE_ALLOWED, PROPERTY_FALSE);
    SET_BLOCK_PROPERTY(blockname, DELETE_ALLOWED, PROPERTY_FALSE);
 
  END IF;

blockname := GET_BLOCK_PROPERTY(blockname, NEXTBLOCK);

END LOOP;
.....

Related:
[Code Sample]loop trough all records of a block