It a horse threatens to kick you then you absolutely MUST punish them for it.
God help any of mine if they even thought about it. In the last 8 years I have only been kicked once (if you exclude a paniced pony), and I'm fairly sure that the pony who kicked me thought that the world was ending due to my reaction which involved alot of screaming, shouting, several wollops from the lead rope onto his side (very very hard) and me chucking a body brush at him (all within the space of about 30 seconds after the kick). He has never ever tried it again with anyone.
He still gives me cuddles and kisses and is generaly a lovely pony but he has never threatened to kick at anyone since then.
The only kick I have ever forgiven a horse for actualy happened last weekend with my new baby pony, the gate swang shut on him as we went through it and he was trapped, he paniced completely and unfortunatly whilst I was trying to free him from the gate he landed a kick on me. I've got the most enormous bruise on my leg, but he was laashing out at the gate that was trapping him and not at me, any telling off at that point would not only have made the situation worse but would have been completely unwarrented and would have scared the poor pony witless.
Kicking is a very very serious thing. The wort injury I have had from a kick landed me in hospital after a 11.2hh pony double barrelled me and sent me flying through a post and rail fence, knocking me out and breaking 2 ribs and my collar bone (this was when I was 15). I do know of several people who have been killed by a kick to the head.