This is such a difficult subject, many people would rather ignore it altogether! I've worked with some horses that all I had to do was just get their attention but most horses are not that easy to work with and require some amount of discipline.
Further complicating the issue of discipline is the fact that every horse is different and responds differently to reprimands. Generally horses learn to accept proper discipline, in fact some come to expect it when they know they "deserve it". By this, I certainly am not talking about a frightened horse, what I mean is a horse that knows parameters and understands what is expected of him and what is not acceptable. This all takes time to achieve but is a necessary part of training any horse.
My students often hear me say: Felonies are felonies and misdemeanors are misdemeanors. This means, don't over-discipline your horse for a minor "offense". I also never discipline a horse that is "trying". It is very important that you choose the right discipline and for the right reason.
Some horses respond well to a verbal response and a good kick (no spur), others need much more to make them amenable. Some horses work well with a crop now and then while others work just fine with spurs. In other words, some are hard-headed and stubborn and others are willing and want to please. Some horses can be intimidated with a harsh sound but there are others you must put the fear of God into to get their attention. Some really difficult horses may even need to be "spanked" from the ground while the trainer is riding (this can really help). And, I've had horses that were so bad I just got off and bitted them up again. Once they got their brain rearranged and re-engaged then we'd pick up where we left off, for a much-improved ride.
Whatever the discipline, the more immediately and consistently you apply it, the more improvement you'll get from it. The point of all this is to find out what works for your horse but never be afraid to discipline them - their mothers certainly did! See you next week, JD.