Sheesh, I must be an abuser too draft!!!
I have absolutely no qualms about giving a solid wallop if the time is right. The horse is 500+kg, with teeth, hooves and a heck of a lot of muscle. I am all of 60kg, with a bit of muscle but not much else going for me for self defence. If my horse is going to try me own, I can assure you, I will leave welts if I have to.
Why is a human responding to their horse taking a chunk out of them abuse, when in the paddock another horse would have kicked them to kingdom come?
To me, abuse is nagging the horse. Not giving it positive direction. Forcing the horse to step up as a leader and then when it exerts its leadership on it's human, tapping it lightly or waggling a finger with a squeaky little 'no... bad horsey'. The poor horse has no idea what's going on, and has to make its own rules. Most horses are happy to have a leader, they WANT to be led, not to have to make the decisions. There was a study done somewhere (the details escape my mind right now) that showed the dominant, lead horses in a natural environment end up dying earlier, due to the constant stress that they are under. The more submissive horses tend to survive much longer as they know their place and have direction.
If you need to give a firm whack, then by all means, do it. If it gets the desired outcome, then move on and forget it ever happened. The horse will be happier, and you will be happier.