How to Stop Illegal Mountain Biking
1. Visit the bike shops nearest a trail that is closed to bikes. Ask a salesperson where the nearest mountain biking trails are.
2. If they mention any trails that are illegal to ride, even if they say that they are illegal, use Google to find out where there are reviews of the shop (e.g. in Yelp).
3. Write a review of the shop asking people not to patronize the shop, due to the fact that they are promoting illegal mountain biking.
For example, when I visited Hank and Frank bike shop in Oakland, California, and asked about local trails, they mentioned the famous illegal trail called "Side-O" on University of California, Berkeley property, saying that it is illegal, and "go at your own risk". I did a Google search and found reviews of them on Yelp. Then I wrote a Yelp review: "Don't patronize them! They promote illegal mountain biking (riding trails that are closed to bikes)."