Does the beam output relay pulse when an object (car) triggers it or does the relay stay energized until the object clears the beam?
If the relay stays energized while the beam is broken then altering your code so the buzzer will not arm to sound again until the relay opens should be easy(ish).
If it pulses then does it keep pulsing at fixed intervals until beam is restored? This could also be covered by noting time of last trigger and ignoring other triggers in the interval window.