I do two things, depending on their reaction.
First is I bump the reins, poke them in the ribs and tell them to knock it off and keep moving. This works most the time for me, basically I'm telling them I'm the boss and will let them know when it's time to panic and when it's not.
When they have an object that they get spookier about everytime we pass I change tactics. It's best to do this prior to them getting to the spin and I'm outta here routine. That's when I jump out of the saddle, prior to their spook point, and lead them up to the object so they can smell it.
Biggest point to remember is to not tense up when approaching the spook point or when you think they might spook. This actually reinforces their idea that there really is something bothersome there. The second, don't stare down what you think they might spook at. They are watching you and looking where you are looking (****, sometimes I wish I had the ability to see in the same way they do). Keep your head pointed down trail and just move your eyes if you think there is something that might spook them.