Have the other chicks drawn blood? It'll be worse if it gets that far. If you don't have one already, make sure your brooder light is red. It makes it harder for them to see blood and seems to calm them a bit.
If you can figure out a primary agressor you can put it in chicken jail or isolate your little victim. If you end up doing either of those things it seems to work best if you can "fence" off a corner of your brooder so the chick in solitary can still see and be seen by the rest of the flock. Makes it easier to re-integrate later.