Trigger before insert

Hi all,

===================================
CREATE FUNCTION trigger_test_func()
RETURNS trigger
AS '
DECLARE
cnt int4;

BEGIN
SELECT INTO cnt COUNT(*)
FROM table_test
WHERE ip = new.ip;

IF cnt > 50 THEN
-- THERE THE "INSERT" HAS TO BE STOPED
END IF;

RETURN new;
END;'
LANGUAGE 'plpgsql';

CREATE TRIGGER trigger_test
BEFORE INSERT
ON table_test
FOR EACH ROW
EXECUTE PROCEDURE trigger_test_func();
===================================

How could i stop Inserting record into table by some condition?

Thanx!