We can and should reach whatever conclusions about people we wish. But we should be very slow to fail to observe and accept new evidence about them.

Excluding people from discussion may screen out their nonsense (or at least the things you thought were nonsense), but it also prevents you from discovering that you made a hasty decision. Once you've started ignoring someone, you can no longer observe what they say - and possibly find that they're smarter than you thought they were.

It's worth acquiring new data even from those you've discarded, at least once in a while.