If your horse Ever threatens to kick he must believe he's going to die for it. I believe is was John Lyons or Monty Robert who said something like "when they act aggressively you have 3 seconds to make them think they're going to Die! Don't touch their face, remember you want to kill them not blind them!" Acts of aggression are severe signs of disrespect and need to be nipped in the bud. You are her herd leader, groundwork will help but it won't stop her from kicking the little kid who knows nothing who happens to run behind her. She needs to know that around humans feet stay on the ground. Period. I like the broom idea cause you can't do any real damage with the bristles but it certainly smarts well enough.