How old are they, what are the exact dimensions of the brooder? What do you have them on for feed, what is the protein content, what is the mill date of your feed? What is the temp in the brooder at the heated end, and at the non heated end (if they are still getting heat). What are the breeds of the others, as well as the one that is getting picked on?
Then you have no choice but to separate. Might try putting the one chick back after it is larger and introduced gradually. There are several threads here about introducing new birds. Good luck!