I'm really torn on this one. You don't want to advocate abusing animals, but, as a few people have stated, sometimes the only way to remind a horse that it can't be dominant is to act like another horse.
I was leading a 2 year-old Appy mare back from the pasture once. She was the snottiest horse I'd ever met -- no training whatsoever before my friend bought her. Getting her to come in from the pasture was almost impossible (I resorted to a version of the Horse Whisperer and chased her around the whole g-d thing whirling her halter rope above my head like a propellor until she actually did what they do on tv: stopped and gave up). Anyway, on the way back, with no apparent provocation, she bit me on the bicep. It hurt like hell, but it was also dangerous: we could not have therapy horses thinking they could nip people when they felt like it. Without thinking, I whirled around and slapped her fairly hard on the neck. She started a little, but then she immediately dropped her head and "got in line." She never bit me again.
Horses have incredibly thick hides. There's a huge difference between an open-handed slap and a punch, or between a crop on the haunches and a crop on the face. Physical punishment can cross the line and should always be used as an absolute last resort, but there are times when you have to nip a dangerous behavior in the bud before someone (horse or person) gets hurt. If you know how to do it, a well-timed smack can be a reminder to behave: it doesn't hurt, but it startles them and reminds them to cool down a little. They're huge. They can kill you. I spent two hours in Havana once working on getting an abused horse to trust me enough to touch him, so I'm the last person to advocate wantonly using physical force on an animal. But sometimes, you have to act like a horse...and anyone who's seen a bunch of mares in a pasture knows how that works.
