Research on bullying completely backs this up. Clear boundaries. Looking them in the eye and telling them to stop. Having other people who support you also helps. It’s science. (And if a child is not confident enough to do it, other kids can play that role and be as successful)
"Trump always chickens out" isn't just a funny meme. If you look at the times he backed down it was after even the slightest pushback. I am amazed that the mythos of Trump has survived any amount of time in the American political arena.
I learned that in 3rd grade when I fought to a draw with the school bully. He left me alone and found easier targets.