The best thing you can do is get your horse used to the noise. And Sailor, I find loud ATV/Motorcycles way worse than any mountain bike.
As suggested, Borrow a ATV or Motorcycle and drive it around your horses every day. Start off my by letting you horses follow it. After they get used to it, you can follow behind the horses.
I have an ATV and drag a harrow around the corral to loosen the soil and break up the roadapples. I pull a small trailer and load the manure up and haul it off. Sometimes I even deliver the hay in the trailer. So my horses hear and see the machine on a regular basis. When you ride around them, don't go gentle. Goose it!, Make noise with it. Let them hear it at it worse. So when a jerk does that on the trail, It's just like you at home.
Its the same principle as teaching your horses to be calm around people shooting guns. When I'm hunting and I see an elk, I bail off and pull the trigger, I don't always have a hitchin post, tree or something else convienent to tie my horses up to. I don't want them getting suprised when the gun goes off and running away. It's ok that they can be startled, Just not spooked enough to run.
Regardless of whether this is inconsiderate of the ATV riders. It's your responsibility to teach your horse to spook in place and look to you for leadership. If it's not a motorcycle on the trail, it will be a truck passing you along the side of the road, the PA system at the fairgrounds, the marching band in the parade or strange Llama and some hikers in the mountains.