Up untill a few months ago we had horses in the back yard. We flew foamies over their head all the time. These horses will get used to the noise and after a while will ignore them. Frankly if a horse will spook at a plastic bag, that is a horse i would not keep. there are good horses and bad ones, any horse that can't adapt to their surroundings is not a good one.
Btw who do these horse people think they are, that all that public land is their own private playground? If i was over the parks dept. i would just tell them that if they can't share, they are out.