MIAMI — The Miami Beach City Commission voted Wednesday to impose midnight curfews and the 6 p.m. closure of the beach in South Beach during two weekends in March in anticipation of a flood of spring break visitors. City officials outlined a slew of measures aimed at shutting down the party atmosphere that in recent years has seen large crowds pack Ocean Drive and led to shootings that have made international headlines. Wednesday’s vote represents a preemptive plan to implement curfews and close beaches ahead of the arrival of spring break crowds. In past years, the city has only implemented c…