At what point do "bad apples" irrepairably taint an entire group?

[Leaper]

So I was reading this article on a libertarian blog on incompetence in the NY crime lab. Necessary background: this particular author is very concerned with (and points out a LOT of cases of) police misconduct, especially when said misconduct is punished with denials of wrongdoing by superiors, slaps on the wrist, and/or promotions.

Some quotes (all modded up) by commentators:

You have to be an evil, sick person.

Hell yeah! What BamBam said. A special mentality alright, that being one in the same as a sociopath.

These are not isolated examples. Commenters to this blog do NOT like the police as a rule. As far as they're concerned, since the bad cops are never punished (and are even defended from wrongdoing by their superiors and peers), with a steady drumbeat of horror stories, each more outrageous and horrible than the last, the entire profession is tainted in their eyes. It reminded me how this can happen to groups in general, like Christians, to the atheists on this very board, people of an opposing political viewpoint, and so on.

Which brings the question: at one point do all members of a group become suspect based on the actions of a subset? When does the reputation become "deserved"? Is it "right" that it happens at all?