May one stop the ringing off a mechanical alarm clock? Why?
It is permitted to stop the ringing of a mechanical alarm clock because there is no melacha involved by turning it off. While we may not mechanical noises on Shabbos, (hashmoas kol) turning the alarm off is only stopping the noise and not making it. It is also not considered moving muktza because a mechanical alarm clock is considered a kli shmilachto l’heter.
Shmiras Shabbos Khilchoso 29-32