I had always the opinion that a Capt. should never leave the cockpit. Never.
The cabin crew has a Purser who is the person trained and entitled by the company to access these situations.
If the purser is having problems to dela with it , and I`m still on ground pax stays on ground. I don`t even see him. Just call the police. And they take him/her.
Once , bound to S.Tomé and Prince , one became violent , I asked two mail flight attendants to retain him.He finished the flight laid in the aft galley.Police upon arrival.
Another flight. Flight was delayed 20 hours.A group of about 10 guys entered shouting and saying they would be a motin on board. Purser to came to cockpit. I told her to tell them to sit down, shut up and behave as everyone else. They refused and kept disturbing. All group stayed on ground. Never saw them.
Cabin crew are trained to deal these situations. If they are not managed to do it , it means the situation has exceded the reasonability.Time to call police.
The Capt has nothing to do in the cabin.