I am not sure what I would have done without actually doing it, but I have re-trained many horses with problems similar to this without a spur or whip - ever. All I did was train them to the T in the arena, and then take them on the trail.
I suppose I have just seen too many accidents and bad outcomes with spurs in situations like these. I am used to spurs being used in dressage, reining, pleasure ... things with subtle and precise cues. Not abrupt, seemingly rough cues.
I have no problem kicking a horses ass down a trail, but I have never used a spur to help me do so, with the way I kick (sometimes, I am a naturally refined rider and prefer showing) they would end up with a hole in their side. I have a well trained eye and mind on the back of a horse and have a good ability to tell when and which way they are going to try to turn around or get stupid and I occupy them before that. I hardly ever have to encounter this on a trail though, because they are very well trained in an arena first.
You can tell your horse you mean business without spurs, can't you?
Dahlia <3 1/4/2012