Remember - aggression breeds aggression. That's not the point we're after. We're after letting him know that that type of behavior is NOT acceptable. We do this assertively and not aggressively. Obviously if we kick with pointed cowboy boots in the gut, we CAN and DO inflict physical pain and harm.
Hooves are flat - broader area of contact, less pointed infliction.
Horses that kick each other are well aware of where and with how much pressure to kick for a particular need. Horses instinctively know that a well placed, hard kick to the forehead of another horse will kill it. They also instinctively know that a kick to the hiney end will not do too much more than give a bruise - kicks to the hind end of a horse by another are not intended to kill. Kicks or blows to the face are. However, the harm we do when we overreact is harm to the psyche of the horse and our relationship with that horse.