Legal Question in Civil Litigation in California

It does not have to be notarized; it is still binding upon him. Even if a document is not signed if the parties conduct themselves as though they are bound by the terms of the agreement then it can be enforced as though it were signed. You could get an injunction against him. You could see if the police will cite him for creating a safety issue but they may decline because it is private property. The cheapest way, aside from asking him why he now is breaching the agreement, is to have an attorney write a letter to him [about an hour of work; I charge $150 per hour] or sue him repeatedly in Small Claims court. If you are going to sue, first give him a written demand and then for a month of so take pictures of his violation, including the date and time, and send him several follow up notes as to what he is doing and why it is unsafe.