Ok, so, my girlfriend and I both have new horses. We board together and ride together. Both out horses are around 10. Mine has been on lots if trails and hers never has. Hers does great and he is pretty bomb proof, so is mine. I guess not bomb Proof, but they don't bolt or buck. We've gone on about 20 long rides together other than in the outdoor arena, these20 rides are mostly on the roads around our pasture area, the last time we went out there was someone shooting a gun far far in the distance (there is no hunting areas so I'm going to assume it was a cap gun or target practice at a farm yard and we were no where near it), anyway, both our horses spooked, now our spook is a jump and freeze spook. That's about it. My girlfriend was like, oh the gun scared them, maybe we should go the other way. I told her its fine, lets keep going. Out of no where I yelled "BOOM, BOOM, BOOM". Both horses froze and I laughed, I told my girlfriend that if they were scared of the small gun sound then we need make a louder sound to show them its ok. We walked along some more and I did it a couple more times, the horses did nothing these times. About 10 mins in again the gun went off, our horses did nothing. After my girlfriend caught on to me barking the boom sounds I did give her the look before I did it the last couple times so she was aware I was about to do it.
So, what would you of done? Maybe what I did wasn't right, but it worked and it's what I could think of for despooking at the time.