I suffered 5 years of bullying at school; and tried everything to get it to stop, without success.
Until one day I just got fed up, walked up to the main tormentor in class and, unprovoked, hit him really hard in the mouth (I think I broke his nose, I'm not sure). Was in weeks of trouble (with the staff) :) but once that was out of the way no one touched me again.
I've never outright recommended this approach to anyone; and indeed it's probably not the right thing to do all the time. But some sort of short-sharp-shock can work just as well (so... barging them to the ground etc.) as long as it is sufficiently hard.