Sometimes, when a horse comes in for training with a specific issue and I can help change that, the owner will be pleased and ask me "How did you get him to stop?" My answer is: I told him to stop. I don't know if it's owners not being confident or if they are trying to find a root cause or if they have learned to accept certain behaviors from their horses, but most of the time horses continue with unacceptable behavior because no one tells them they shouldn't.
There are behaviors that stem from a horse's personality that can not be changed. Training can help a horse be a better partner but it can not change a horse's personality. There are exceptions; the horse that is normally mild mannered but has become aggressive due to a fearful owner or horses that have come from abusive situations and have become fearful or withdrawn can be rehabilitated. In general, a horse's personality, that it is born with, is what you have to work with, not try to change.
The most common behavior problem I see with horses that come to my stable, is that the horses aren't aware of boundaries. Either they've never been told or they've been allowed to slip into bad habits with regard to personal space. Fortunately, it's one of the easiest concepts for a horse to accept. As long as the handlers are consistent and fair, that is. Nothing is more confusing for a horse than inconsistency. One horse that comes to mind immediately is Mod. Mod is an enormous gentle giant of a horse but because of her size and gentle nature no one had ever told her she needed to look out for people. She wasn't mean or aggressive or bossy but when she walked somewhere, people got out of her way. In a single lesson, Mod learned that she ought to be more aware of people in her life! When I told her she needed to stop when I did and that she couldn't come within an arm's length of where I was, she said "ok" and accepted that. Whether it was her size that intimidated people or if people just assumed that because she was so big, she couldn't be stopped, Mod had learned she could pretty much go where she wanted and people were little more than speedbumps.
I told Mod, No you can't, and she was okay with that. Horses are like that. They just want someone to be in charge. If the person isn't going to do it, then the horse has to do it. Horses don't understand that sometimes, we want to be in charge and sometimes, they can. That's a disaster recipe. The person has to be in charge 100% of the time that they are in the horse's company or the horse just doesn't accept the leadership. They will not put up with wishy-washiness!
Saying "no" isn't all we have to do, we have to back it up with what that horse should be doing. An example is the horse that keeps moving around at the end of a leadrope when he needs to stand still. It is not good enough to stop him from moving, you have to put him where he is supposed to be. If the horse takes three steps forward and you tell him to stop, he's still accomplished his goal of moving forward. If you tell him no and then put him back where he belongs then he understands he's not supposed to walk forward. Just stopping him says, that's far enough. It doesn't say, stop walking away. If you want him to stop walking away, he's got to be put back where he was, thereby making no progress.
So how many steps does the horse get to take before you tell him no and put him back? Often I will see people letting the horses take a few steps or make a couple of circles around them before they finally say, enough. When I work with horses, I try to tell them no when they start to think about moving. They tell you they're going if you pay attention. They lean. Or they check to see if they can move just one hoof. If that one hoof goes uncontested, they move another one and another.... Before you know it, they've left town.
It takes effort and consistency and that can be tiring or boring but in the long run, it's worth it. Eventually you will rarely have to make a correction because your horse just expects and accepts your leadership. For those horse handlers who feel like they are being to bossy, keep in mind that 23 hours out of the day, your horse can do whatever he wants to and go wherever he wants as fast or as slow as he wants. For one hour a day, in your company, he can be told what to do. I'd be pretty content if I only had to work an hour a day.