create or replace function skip_insert() returns trigger language plpgsql as $$
begin

  return null;

end $$;

create or replace function raise_exception() returns trigger language plpgsql as $$
begin

  RAISE EXCEPTION 'UPDATE changing row values not allowed';

end $$;

CREATE TRIGGER mytrigger ON records INSTEAD OF UPDATE
    WHEN old.PriKey = new.PriKey and old.Foo=new.Foo and old.Bar=new.Bar
    FOR EACH ROW
    EXECUTE PROCEDURE skip_insert()
CREATE TRIGGER mytrigger ON records INSTEAD OF UPDATE
    WHEN old.PriKey = new.PriKey and (old.Foo!=new.Foo or old.Bar!=new.Bar)
    FOR EACH ROW
    EXECUTE PROCEDURE raise_exception()