Agreed with smrobs. I would never strike a horse who is scared - I am not opposed to physical discipline, but I reserve it almost entirely for situations such as an obstinate colt biting or kicking. Horses are bigger then us, and unless you're really ripping into him with a licking for doing something dangerous, constantly slapping them is only going to annoy them and really not teach them a darn thing.
Whether I'm in the saddle or on the ground, I personally act like nothing is wrong. I don't tense up when I see things they "may" be scared of, and I don't bother looking at things they "may" be scared of. If my horse tenses up and bolts on the trail because a dog scared him, I quietly ask him to whoa and continue riding as if nothing happened.
The ground is much the same. If they're being blowy or snorty over something, I simply walk by the object without giving it a glance. I also prefer not to coo or talk to my horses in scary situations - by speaking softly and encouraging them, you're actually setting them up to believe this is a GOOD thing to be doing. They learn that spooking means a good reward.
If they're being absolutely ridiculous about something, I'll set up camp and start working them there. Ground manners, lunging, working their feet, anything to get their attention on me and off whatever they think is scary. It never takes long for them to figure out if they pay attention to me instead of the flapping tarp, they get to stop working so darn hard.
Good for you OP, in that you rarely experiences spooks. That tells me you are a confident leader, because really, the best way to deal with spooking is to prevent the spooking. It's amazing how easily this can be accomplished just by the owner/rider having confidence and not playing into the horses silly little fears. In my experience, a horse will very very rarely spook at little things like garbage bags or colorful pails if the rider/owner keeps themselves calm and unconcerned. Most people don't even realize that they're tensing up and looking at the object themselves and then want to smack their horse around for it.