Acceptable discipline varies from horse to horse. I trim a couple of extremely stubborn/dull cold blooded horses that probably wouldn't notice if you dropped a grenade under their backside. But I also trim a lot that are sensitive and more reactive. The same discipline for both styles of horse wouldn't be appropriate.
The Fjords and Hafflingers I do will try to lean and lay down on me so I use the point of my elbow firmly planted in their side. They figure out pretty quickly that the sharp elbow digging into their rib isn't fun so they stand back up straight. I don't jab or slam them with it. I just tuck my arm in close to my body with the elbow toward the horse and when they lay into it, they get some uncomfortable pressure.
The hotter horses I do like TBs and Arabs are usually good with a "Stand Up!" and nothing else. Each horse is different.
When a horse is truly being belligerant, no attention span, putzing around worried about the other horses or getting too antsy, I either have the owner do some ground work for a minute or two, or I do it. Step back, over, come up, whoa. It gets their mind back on the task and makes them understand that the humans have the ability to move their feet.
Some youngsters or ones that haven't been handled much need distraction with a hay bag hung on the wall, or they need to go for a walk down the aisle and back after each hoof.
The take home message is that at the end of the day:
1. I can't get hurt.
2. You can't get hurt.
3. Horse can't get hurt.
4. Horse has to be calmer at the end of the trim than at the beginning.
5. Everything has to be positive, even if discipline was needed.
6. Everybody has to remember that discipline and punishment are two different things. Depending on the horse, humane and compassionate discipline can be administered professionally and safely but in my opinion, punishment is NEVER the responsibility or right of the farrier (or vet). I personally define punishment as hitting, screaming (versus a stern word or the horse's name), jerking on a chain, or otherwise harassing, scaring, or inflicting severe pain to the horse.
It takes me 20 minutes to do the average trim, but I allow 30 per trim in case there are issues. If the time used goes beyond 30 minutes then I either charge an additional fee for my time, or I reschedule that horse if I have to get to another barn. This prevents me from feeling rushed or stressed because I'm spending too much time, will be late for the next appointment, or am not getting paid for my time.
Also I know my limits on how many horses I can treat/trim/see in a day without getting too tired. I don't over-book myself if possible, and if a day is particularly busy, I put the "worst" horses at the start of the day and the best ones at the end of the day. So for instance, I'm not going to add a new unhandled 2 year old to my last stop of the day when I'm tired and just want to go home. But I will schedule that horse first thing in the morning and I will schedule other apointments accordingly so I have double or triple the time allotted in the book for that horse that will undoubtedly be a problem. Yes, you'll pay for that extra time but I promise it will be worth it