Pinching may work if done properly. Smacking in the face leads to head shy and other issues. Make a fist and punch him in the mouth next time.
That sounds logical
You need to understand the psychology behind what you're doing before you do it, if you want to be effective at training. That is, understand how the horse thinks & understands what you're doing. When talking basic behavioural stuff, it's not that difficult because we all(whatever species) learn in effectively the same manner. A major difference between horses & humans re training is that we have a detailed language to understand past & future events, abstract ideas, etc, whereas horses only really learn from *instant* association. For eg. If you smack, punch, kick... whatever a horse in the face, body, leg... wherever & it happens consistently at the time of a behaviour you want to discourage, that *can* be effective(still not necessarily without 'side effects'), but if you punish a horse(in whatever manner) after the event(even seconds later), inconsistently or in other ways they don't understand, then you're likely to make him 'shy' of you, 'angry', reactive... more playful(in ways you don't want to play)...
and I can't just wait until next time he bites me and then hit him in the face
Yep, agree that *well timed* punishment can be an important *part* of changing 'bad' behaviour but is rarely effective(or fair IMO) if it's the only tactic. Understanding why he's doing it in the first place & changing his motivation is important, as I tried to get across, as is teaching *good* manners which will reduce the likelihood of the 'bad' behaviour. I would focus on teaching him 'good' behaviour & just be prepared to punish in case of 'emergencies' where he forgets his manners(or you forget yours before he's learned they're to be accepted
I'd focus on teaching him it's in his best interests to allow you to be in his space, etc, no matter whether he's eating or not. Teach him what works for him is to have a pleasant expression on his face, keep his nose in & down when close to you, keep his head out of your personal bubble(you need to decide how big that is & be consistent about it) at all times.