What Endiku said. I've been booted a few times. Never had a broken bone because of it, but I know it hurts! And I know that I won't even tolerate THREATS from my horse or anyone else's now. And I don't care what the owners of any other horses I might handle have to say about it, if a horse threatens to kick me I am going to get after it just like the alpha mare in the herd would.
Any dangerous behaviour like kicking, biting, striking or rearing WILL result in one hard sharp whack from me, and I do not care if the owner of the animal is against hitting. I had a biter. He bit me ONCE. Never tried it again... on me. But he bit Mum many many times because she doesn't believe in hitting horses and just made him work around her in a small circle. My nearly four year old Thoroughbred kicked me once. Regretted it. Threatened on many other occasions out of fear but had well and truly gotten over her fear and was just being a witch the time she actually lashed out and got me, so where I had been patient and just ignored the threats so many times, I belted her for actually kicking me. Once, hard, and she didn't try it again.
IF YOUR HORSE TRIES TO MISBEHAVE THE SAME WAY AFTER BEING DISCIPLINED ONCE, THE DISCIPLINE IS NOT FIRM ENOUGH.