Any force necessary. I have kicked a few dogs coming after my horse, a pointed boot to the ribs works pretty good...riding crops work well too.
I typically will charge any dog that is coming at me while I'm riding. Jake doesn't spook away and will run over any dog that gets in his way. I was once riding with a friend and we were charged by 3 dogs that came out of a garage. I turned Jake towards them and set him off at a trot towards them, they backed off and went for my friends horse, who sadly dumped the her and took off down the road. Luckily I managed to block the dogs and chase them back and the mare came right back to her owner who had pretty bad road rash.
The owners ended up supplying first aid, and helped drive me back and forth so I could take both horses back to the stables and took my friend home while they promised to never let the dogs off leash again. So I never got mad about it, they learned their lesson.
I have dogs that I take on trail myself. They never bark, chase or even go near any other horse than my own. If I see other people with horses I go off the trail, let them know I have dogs with me (so if their horses are spooky), both dogs sit next to me and I let the other people pass. So I don't mind well trained dogs around horses in the least.