What you did was right. I certainly would have done it, because if in any time a horse becomes a threat to others, it MUST be disciplined. They ARE powerful animals and can kill a human without even realising it. That is why horses must be told when they are becoming a threat so that they don't go so far as to hurting someone.
These people who think a horse has to be perfect at a show and think that if someone so much as raises their voice to a horse scream "animal abuse" need to get their heads read.
Yes, there are people out there that unnecessarily hurt/scare their horses. But they mustn't just assume that if a person is smacking their horse it is classified as abuse.
People must just learn to assess the situation. If they can't do such a simple thing and think bad things of you, to heck with them. What you did was maybe even for their safety, and if they don't realise it that is their loss. Do what you think is right and don't be concerned about what other people may think of you. That's just their problem, not yours.