One which works rather well is to offer your clenched fist to be bitten, when brutal brat bites push your fist into his face thus pressing his lips onto his own teeth which quickly becomes uncomfortable. My friends had a boy who used to slap or punch people in the groin, when he came at me the second time I stuck out my index fingers as if they were pistols and prodded him in the stomach, and then the ribs when he turned to hide his stomach from being the target. At each prod I made the sound of a shot, as though we were playing cowboys and indians. He came towards me threateningly a couple more times but as soon as I stuck my fingers out he veered off, and took his anger towards his father who got a smack in his goolies, which gave me a sly smile. If the protagonist feels pain they don't come back for more, but if they do double the dose:))