Celeste's approach works with my Dutch Warmblood. He's fairly "hot" and spooks fairly easily. Maybe 75% of the time I can tell he's in Spook County because of the way he's carrying himself, but about 25% of the time, there's no warning at all. He'll be going along in the ring at the trot or canter, next thing I know he's leaping sideways and wanting to run.
I find that if I sit VERY firmly, as if I am requesting a full stop with my seat, and take up a lot of contact with the reins - flexible contact, not making a brick wall with them - it will keep him from bolting long enough for his Thinking Circuit to kick in and stop the panic attack. Last week we were out on a trail ride with another wiggy horse, and just as we got back to the barn my horse missed his footing, spooked, and tried to bolt, at which point the other horse spooked and did bolt, dumping his rider. This set my horse off again, and I had to do the circle/single rein stop, but I was definitely keeping a lot of contact the entire way.
OP, I have not found a way to keep my horse from spooking in the first place. Plenty of times I can't ever figure out what made him spook in the first place - some tiny noise made by a chipmunk, or an unexpected stick, no telling. I try to keep him desensitized to the obvious stuff, and for the rest of it, I pay attention to his body language that tells me where his head is, and I pay attention to the environment for stuff that I know spooks him.
Sometimes if I make him focus on me by working him - lots of transitions, that kind of thing - I can get him out of Spook Country. For me, it's always worth a shot. I have also found that sometimes even that doesn't work, and then it can make the problem worse...so I'll try it a few times, and if it's not working, then I go to Plan B. Plan B could be something like getting off and lunging him, or it could be riding in the arena instead of riding on the trail, but it is NEVER getting off and letting him go back in the barn or paddock.
I saw a piece of advice above that if your horse dumps you like that, make sure that he works good and hard when he gets home - that sounds reasonable to me. Whatever else happens, he shouldn't be getting rewarded for this behavior.