When I talk with certain people, I perceive a very clear though subtle message: "I am the good one, I am the one who is right, and you are not."
One can be wrong in their perception, so I do not act right away.
Beyond a certain threshold of interactions confirming the above trend, I consider it safe to assume that the person in question does not deserve to be considered a good person.
So I block them.
@Eidon
And, what about if you were (hypotetically) that person you describe?