I think I need to qualify my "smack the horse in the side of face".
I never hit my horses in the face either. However, when you're in the saddle and the horse already has it's head 2/3rds of the way down for grass, a smack at the face isn't going to be a legitmate smack in the face because you can't reach far enough to make a hard connection.
The horse generally gets lightly tapped and should associate putting its head down with something coming up out of the grass and biting it - making the whole experience very unpleasant.
Years ago, a good friend took a serious nose dive off her horse because she would not correct him from eating on the trail. She was about "5-foot nuthin', might have weighed 100 pounds and rode an English saddle.
We were at a fast walk, when her horse suddenly decided he wanted grass, lurched his head down and she went clear over his head. Thankfully nothing more than a few bruises that included her ego.
It became a real "bear" to correct that horse and smacking him on the side of the face, while sitting in the saddle, was the only thing that worked. At 100 lbs and with a short reach, the only damage done to him was HIS ego and the fear something in the grass was going to bite him