Ok, I'm going to suggest something different..
A read recently where a noisy dirt bike was playing nearby and disturbing a neighbor. He was going to do something and then realized that if he was 40 years younger, he would be out there talking to them, finding out more about them and their bike, not trying to chase them off..
Perhaps you could stop and talk to some of them, explain kindly your situation and them to help you solve it... could we move the trail a bit to lessen the noise problem (as you tried)? Approach them from a how can we make a few simple changes to allow the area to stay open. They probably don't want to lose the riding area, so if a few changes can be made to stop it from getting shutdown completely, it benefits all involved.