These occurences seem to come in waves. Back in the 90's I was involved in "having a word" with 2 idiots aggresively intimidating young female CC on 2 entirely separate flights. On both occasions alcohol was involved and after myself and other agrieved passengers explained that their behaviour was unacceptible and liable to end badly for them, they calmed down and there was no further problem. In the same decade I also witnessed a brawl between several scumbags, again fueled by alcohol, on a flight in the US. Since then, despite having made more flights than I care to mention, I have seen no real trouble at all.