There would seem to be various categories of "abusive" being discussed here.
If an individual, or group, wish to behave like tossers, then in a free society, they should be able to do so, as long as their behaviour doesn't physically encroach on, or harm others. It is impossible to legislate against some people being unpleasant idiots, they can only be shown up for what they are. In the case of Wildweeble's unfortunate incident, then I think it is perfectly reasonable for the other pax to ignore her plight, as long as it remains only verbal. The best course of action has always been to ignore such outbursts and I would do the same thing. However, if she, or her children, came under physical attack, then it is a different matter and I am sure many people would feel constrained to intervene, myself included. There is a huge difference between verbal and physical assault.
As for someone who refuses to obey the instructions of cabin crew, then it is down to the cabin crew. Again, IMHO, it is unreasonable and indeed, undesirable, for other pax to get involved. If the cabin crew are unable to contain the situation and feel the need to ask for assistance, they can always ask either the police, or in extreme circumstances, other pax for assistance. I would be happy to assist cabin crew if requested by them, but I would feel very incomfortable getting involved unilaterally. As F3G pointed out, you then become part of the problem.