Roof Top Gate

Our condominium building is a row house (a block of same size houses with rooftops that are of the same heigh and anyone can walk across them) and we are surrounded by two rental buildings. These two other buildings have become party houses, where they host illegal rooftop parties and the parties are known to carry onto our roof without our permission. We recently replaced our roof and we don’t want it to get distroyed by careless individuals who are not at all invested in our building since they are just renters carrying on our roof. We contacted the landlords, but nothing has happened. What makes it difficult is that our enterance to our roof is through a hatch with a two story submarine laddar attached. So we don’t have an easy access to yell at people to get off our roof. We called the police several times and the party just starts off once the police leave. We were thinking of installing a gate on both sides of our roof top to prevent people from accessing our roof. Does anyone know if this is legal or not? We are also open to anyone’s suggestions.