I feel your pain. The best advice I can give is to accept that tonight is a lost night. Tomorrow is the time to go talk to the owners about the situation. Be calm, rational, and even if you are drawing a line in the sand, make it sound like a negotiation.
But first, start a journal. Record tonight's events - details of dates, times and activities. Keep this journal faithfully, and include all of your contacts and copies of any written communication.
Try to not be the cranky neighbor, but stand up for your rights. You were there first, and this is one of those things where their property rights do not override yours.
Law enforcement will tell you that if you do want them involved, they must witness the actions. So don't wait until they are done riding to call. If you do encounter this and the activity ends before law enforcement arrives, video and your journal will help them to at least make a visit.
Good luck! I hope a simple neighborly talk can provide you with some sense of comfort on your property and not ruin the possible friendship.