It's sort of like beating up the neighbourhood drug dealers in an attempt to pursuade them to stop selling drugs. It's not legal, although they're unlikely to turn you in to the cops, it's only ethical if you think that the ends justifies the means, and, most likely, all you'll end up doing is getting them to move on to new victims making them someone else's problem.

But, for those with a mildly violent streak, it'll feel good while you're doing it ;-)