How do you stop a foal from kicking at humans?
A foal that kicks at people is establishing his dominance in the relationship. You have to reprimand the foal to set the boundaries for acceptable behavior. Other horses will reprimand a foal by nipping it to discourage kicking. As the foal gets older, the mature horse will bite the foal and run him off if he kicks. Eventually, the dominant horses will do whatever it takes (biting, kicking, attacking) to teach the young horse the appropriate manners. You have to establish the same respect from the foal the other horses will demand.
Put the foal in a round pen and use a rope, lunge whip, or training stick to drive the foal around the pen. If it stops or kicks, move quickly at the foal and scare, or tap the foal to punish it and make it move forward away from you. Make sure to have a whip or stick long enough to keep yourself out of kicking range. The level of aggressiveness needed will depend on the foal. Hopefully, you will never have to punish the foal, but you must do whatever it takes to get movement. Periodically turn the foal around and go the other direction.
Be gentle with the foal unless it kicks or threatens you. As the foal starts to slow down, stop and step back to see if it will come to you. If it comes to you, pet and rub it for reassurance. Anytime the foal walks away or threatens you, get after it and make it move quickly. Try to keep the round pen time to a minimum each day, but routinely practice this method. In time, the foal will find that being your friend is good, but threatening you is not acceptable.