Marin County is cracking down on several Stinson Beach homeowners who erect makeshift fences in front of their multimillion-dollar homes in summer months, when high visitation can mean unwelcome guests. Beachfront homeowners say the barriers have long been put up to protect against trespassing, but according to county code enforcement officers, the fences are illegal—even those that lie entirely on private property. An anonymous complaint filed on Aug. 13 with Marin County’s Community Development Agency…