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!