I was going to ask isn't it different when handling a stallion?
I heard it was bad to hit a stallion as this can make them more aggressive? Obviously people do it to discipline them but I was told you have to be really careful..
In theory you shouldn't hit any horse unless it's an absolute last resort. If you go straight to hitting to correct a misbehaviour you're likely to make a horse sullen, withdrawn and upset - and stallions will usually fight back because they have more testosterone than geldings or mares.
I'm not into natural horsemanship but I am a BIG advocate of the adage, "Do as little as possible, but as much as necessary."
So, if I want a horse out of my space (for example), I ask very softly first, in the body language equivalent of a whisper. If I'm ignored, I ask a little louder. An ask. If that's ignored, I tell. Then I demand. And then I promise. And "promise" is BIG - huge energy, huge dominant aggressive body language, and yes, if needed, a crack across the chest (or butt) with a dressage whip. 99% of horses will react to the final "promise" phase with a big "WHOA OKAY Imma get out of the crazy lady's space she means BUSINESS" - which can take the form of quite a violent spook - but 99% of horses will then not need me to go that far again.
Edit; it's not something you want to do in front of people though because it looks quite horrible. You want the horse to honestly believe the devil is at his heels if he refuses to respond to the lighter cues. The difference between this approach and abuse is that in this approach you have given the horse plenty of opportunities to respond to gentleness first, whereas with abuse, people just go straight to thrashing the horse if it doesn't do as it's told. If, for example, I was asking a horse to yield its hindquarter, I start by just giving that part of the horse's body a certain look. With an attitude of "You're going to move and I mean it" - this is the whisper phase, you have not yet given a physical cue, it's all in the attitude. Then you ask by touching the hindquarter and pressing lightly against it. Tell is a repetitive tapping motion for three seconds. Demand is a firm repetitive open-handed slap - not belting just showing that you mean business. And then Promise is all he!! breaking loose. And THE MOST IMPORTANT PART is to COMPLETELY stop and just stand there the instant the horse yields.