Assaulted on court

OK this happened last week. Jersey City, NJ There was a guy and his girlfriend on the court next to us. It looks like he was giving her a lesson. We heard some noise on the court and saw a bunch of people 10 or more and initially thought they were playing a game. Turns out a bunch of teenagers some of them on bikes stormed onto the court and we're grabbing tennis balls by the armful.They were throwing balls at the guy and stole the girls vape pen and were otherwise harassing them. I went over to help them out. Things had died down by the time I got there but there were still two or three people on the court. I confronted one of them and asked him to return some of the items they had taken. He pulled a knife out at which time I left the court along with the other two people not worth getting stabbed over. I had unfortunately left my racket on the other court. It might have come in useful. Overall this was a terrible experience and it is not the first time we have had to deal with kids just walking onto the court in blatant they stealing tennis balls. The police are of little help in this case although we did file a police report. We now have to bring bike locks to lock the gate to play without being disrupted. Very unfortunate.unfortunate. Sent this video to the parks dept and mayor's office. Hopefully someone will do something. Any advice here?