Why does Facher say these people can never testify in the film civil action?

In the movie Civil Action, Facher does not say anyone cannot testify; Facher was the plaintiff's attorney who tried the case. However, the defense, led by Jerome Facher, argues during the voir dire (jury selection) that certain groups of people might have biases or conflicting interests that could impact their ability to give an impartial testimony in a civil action case like this one concerning drinking water contamination of municipal residents.