In my opinion I don't think smacking a horse when they are biting you will make them head shy. Obviously don't abuse them but, if they do start to become more afraid, give them pets and love. To show them that they shouldn't be afraid of your hand or anything but they still need to respect you. I've worked with a stud who has a tendency to bite and the first time he did I smacked him and said "no" loudly. I've never had a problem since with him and we are best buddies.
Hi! I would never hit a horse on the head, because it will become head shy. I like to have my knuckle right at their face and push it away when it comes to bite me. Just ignore. Just keep what ur doing. And if it's a mare, mostly every mare bites. It worked for my mare