When it happens (and if you race long enough, it will happen):
1. Make sure everyone in the field knows there's a hazard. Don't keep it a secret.
Try to neutralize the race if you can. Don't pick this moment to attack.
2. Don't scream at the driver. They're already disoriented. They're more likely to react WRONGLY to what you're yelling.
3. ONE person should take the lead and calmly lead them off the course.

Remember, to EVERYONE else, it's just another day in their town. To EVERYONE else, it's just a road that they use every day.

How many times have you gone around a barricade on a construction site hoping that you could get through? That's what they're doing. No one believes a sign that says "road closed".