With my mare, especially because she is so sensitive, I won't touch her face at all. If she is just getting a bit too mouthy, I too just kind of pinch her lips a bit, and on the couple of occasions that she's actually tried to bite me, both because I was messing with an injury, and she's still unsure about people, I will smack her on the chest or the shoulder, but never the neck head or nose. I also don't smack as hard as I can, a swift tap is enough for her.
My first horse used to bite for whatever reason, and his previous owner would wheel back and punch him in the soft part of his nose, so he started making a game of it, bit the human dodge the fist. He was definitely dangerous because he would take hunks of skin, because he was trying to get in and out quickly, which meant that he grabbed whatever he could. So obviously smacking him just made his biting worse, and made him head shy. It took me 3 years, but I basically just ignored him. He wanted attention, he learned that biting got him attention, so I took that attention away from him. He bit me, I just walked off back to the tack shed, grabbed another brush or whatever, walked back and continued grooming him, and wouldn't even acknowledge that he'd bitten me. At first I definitely wore THICK clothing, but as time when on, he got more gentle, and finally he just stopped one day. I still wouldn't trust him enough to knowingly and willingly turn my back on him and focus on someone else when I was in his reach, but he finally had learned that he wasn't getting any extra attention from biting me, so he may as well stop, and just act nice.