Occasionally, whether from innate temperament or an abnormal level of hormones, you may find yourself with an aggressive chick.
I've watched a broody discipline her little babies and keep them in line by giving them occasional but necessary pecks on the back or top of the head. They let out a startled squeak and fall into line.
When I had a week-old Cream Legbar chick obsessively plucking the down from his (later discovered it was a "he") mates, I tried poking him on the back with my finger. He would abruptly stop. When he tried it again, I gave him another poke.
I spent five minutes at a time occasionally for the next two days watching him and poking him when I saw him start to go for a chick's down. This successfully stopped the bad behavior. He's now a splendid, perfectly behaved three-year old rooster.
I wrote an article on this topic for BYC. It's linked below and has some other tips on preventing aggression in the brooder.