i believe it is ok to discipline a horse, but certainly, this must be done fairly and without using excessive force. Yes, horses do kick each other in the paddock but first they will put their ears back, shake their head, point their hindquarters at the other horse, move towards the other horse and THEN kick. The other horse has plenty of warning and if its silly enough not to believe that the other horse will kick it - then it will soon learn!
Personally, I use a similar principle with my horses. If I want my horse to move out of my space, I'll gently push against her with my hand and she'll willingly move. But she wasn't always this way, she used to prefer to ignore my signal and go on standing. So I progressively increased the intenisty of the signal until I got a response. As soon as she moved, I stopped and left her alone. Now, she knows that if I want her to move, i'll get what I want and so she moves when I ask her politely, before i start getting insistant.
Certainly, disciplining a horse has to be in a way that the horse understands - if it isn't done properly, it does more harm than good to the horse.
When I bestride him, I soar, I am a hawk: he trots the air; the earth sings when he touches it; the basest horn of his hoof is more musical than the pipe of Hermes. ~William Shakespeare