Because I just say "Ow."
I see a lot of horses get smacked in the face over nipping. Some do it to be naughty, but some do it just because they're mouthy... then I see the horses that are getting a nice scratch and they start to "groom" you back, but some people, as soon as they feel those teeth or the lips wiggling, they'll backhand that horse's muzzle without missing a beat. I was always taught to never strike a horse in the face. I'll pinch their lips slightly as a warning if they are play-nipping, and that seems to work.
Anyway, what do you guys do?