Also worth checking out -- many years ago I had a client with a horse who did this and it became quickly obvious to me that the cause was saddle fit. The horse could *stand* the pinching saddle the rest of the time as she used a mounting block to get on, but when getting off the weight on the pommel was too much for it to bear and he would act up/kick/run forward.
I had her give him a couple weeks off to let his back muscles recover/heal, get a better saddle and then I got on and off about 25 times with a holder giving him a treat while/as soon as I dismounted. Once he realized it wasn't going to hurt he was fine without a holder, and I gave him a treat as soon as I got off for a while. Poor fellow, he was absolutely stoic to be as good as he had been about it the rest of the time.
If I had one with a spur fear I would use the same technique. Spread it out over a few rides so the horse doesn't get sore from the repetition if you can, and use a mounting block so you are not pulling his back to one side repeatedly and making him sore that way.
I have had a few horses like this that came for training. Generally, you have to do this a lot. I would suggest maybe examine your workouts. Make sure that you do a lot of relaxation work in the workouts - suppling work etc - a lot of times the work headed up to the dismount can put the horse in the right frame of mind.
Then I do agree with someone else who suggested treats. I dont like to BRIBE horses BUT I understand that by nature they associate fear and insecurity with not having Friends and Food. IE - they were spurred means something unfair happened the horse felt she did not deserve - or he - and it breaks the herd/friend code for horses. I can punish a horse for being outright naughty or dangerous if I know that is where their mind is - and if I punish said horse for that and am right - the horse knows it. A horse that kicks ugly - can be admonished. That horse knows he did wrong. The alpha mare would do the same in a herd. But if a horse gets punished for something untrue, this labels the rider as unfair. And then there is a grudge. Sounds like this horse holds a grudge. A lot of horses would get an accidental spur and be like - OK, maybe that was an accident. SO I would take note that this horse must be trained by someone who knows how to read a horse - so some other accidental thing doesnt become an issue.
We have a horse in the barn like this. But its all good - he has come along nicely. And its just time and time and time of building a relationship - desensitizing - so back to what I said initially - a lot of mounting and dismounting and reconditioning.... I would give a treat when the horse halts - and I would take your feet in and out of the stirrups throughout a ride - I would treat the horse when you dismount. Get the horse used to looking back to you for that treat once your feet hit the ground instead of bolting away.
But I do suggest you try to arrange the horse's training times when you know someone is going to be there. No matter how good a trainer you are, horses still outweigh us and bolting can be pretty dangerous!
update time! at the risk of jinxing myself, i can report that things are looking up. the session with my trainer helped a lot. as a few of you suggested, a big part of the problem was the halt. i realized that he finds halting and standing still extremely stressful. i've been doing tons of walk/halt, trot/halt transitions, keeping him halted for a while, and generally standing around with him quite a bit. after a good workout and once he relaxes/gets tired enough, i've been halting, taking my feet out of the stirrups, petting him all over, etc. i dismounted/mounted about 10 times with the trainer, and - when he was really good - even nudged him slightly on the rump with my boot when i dismounted. he still spooks slightly when my feet touch the ground beside him (for some reason this is shocking), but he's improved immensely in just a few rides..knock on wood! thanks for all your advice! Now it's time to work on everything else ;)
Reward after dismounting
He still spooks slightly when my feet touch the ground beside him (for some reason this is shocking),
If you have his favorite food reward in your pocket, ready to give him as soon as you land on the ground, he will start to look forward to that moment. Be sure to take the wrapper off the peppermint before your ride, if this is what you use. You probably wouldn't have to do it for too many times, just until he is standing and calm as you land on the ground. Then reward him every few times, and then almost never. [I've had great results using food rewards to teach behaviors over the years - starting when I was 11 and teaching our dog to open the rabbit cage and eat lettuce.]
It has been interesting to read all the different responses to your question!