I agree strongly with this.

I was hardly picked on in school because I'd just ignore any teasing... and the first time I let it get to me I stabbed the bully (in class, in front of the teacher) with a pencil and sent him to the doctor.

I got off with just a chat to the headmaster... not even a very stern chat, because the staff all liked me as a good student and disliked the bully because he was a known bully. Doing the same thing now would probably lead to expulsion and charges of aggravated assault. If I had a child today I'd be telling them to make sure they drop the pencil first; using fists would probably work nearly as well, and a suspension for fighting on your school records means nothing in the real world.

It's a bit sinister, though, don't you think? If you're dealing with a vengeful bully, the threat never really stops until you've injured him enough that he will be unwilling or unable to hurt you back later.

The trick is to damage one of them badly enough (albeit non-permanently) that nobody else wants to try. I generally found that picking somebody clean off the floor by the throat and holding them in midair until the look came into their eyes where I knew they thought they were going to die was a good option. Doesn't give them much except a day or two of a sore throat in terms of physical damage but you only have to do it once every two or three years to be marked "not an easy target".