I agree with the above statements regarding hitting - at least in the face. You should absolutely never handle a horse's face in any manner than that which is gentle and kind, regardless.
My horse was previously abused and was very head shy, but he used to constantly "nip" me.. he would never actually bite, but would grab my jacket or shirt with his lips. I started paying really close attention to what I was doing while he was nipping, and realized that it only really happened when I was brushing a certain area or tightening the girth. When he would turn around to grab my jacket I would face him sternly and say NO! in a very loud, stern voice. And lightly tap (not full on HIT) his shoulder. When he was behaving while I was performing the tasks that used to bug him, I would reward him with treats and lots of praise. I've been doing this for about a month and he's gotten ALOT better.
What I would try with this horse is a lot of positive attention. Whenever someone is passing the coral and the horse, have them go up to the horse before he has a chance to bite him and pet him all over his face and body. Make sure they always maintain a dominant posture (directly squared up with the horse) and do not allow him to act badly - he may be trying to establish dominance. I'd also call your trainer and ask for advice - as long as they don't try to or recommend cowboying the horse, they're usually a good source of information and help.