those that are saying the Capt will have no better ability than the cabin crew to control trouble makers are pretty ignorant.
No matter how big, tough and pit bull-y you are, there is always somebody bigger, tougher and more pit bull-y than you are. And there you are, flat on your back in the aisle with a broken nose and fewer teeth than you had a moment ago.
Yeah.... try not to let your preconceptions get the better of you. I'm not talking about going back down there to physically intimidate someone. The simple facts are that as the Skipper you have an automatic authority that most people will respond too. Usually it's people being loud, awkward and uncooperative. Your simple presence is enough to make them recognise they've stepped over the line and to behave. However if your cabin crew advise you that someone is being *physically* agressive then that's a different matter; steer well clear. A simple matter of communication and using your brain.
Actual example: Single mother insists on repeatedly letting 2 year old toddle around the Cabin (during taxi, initial climb and a patch of turnbulence. Cabin crew return the toddler and tell mother to secure it. As soon as they leave her, mother releases the kid again. Crew advise me. I assess zero physical threat but real risk of unsecure cabin during approach/landing so I amble back and have a quite, polite but very firm word. Problem solved.
Also note that I am writing in the pre 9-11 context. As I very clearly stated (but you chose not to quote) in these days the risk of it being a decoy means that no-one should leave the flight deck during a disruptive passenger incident. The industry has quite correctly responded with SOP changes and warning letters for handout etc.