Here is the quote from Miller in the text of the play:
“In Proctor’s presence a fool felt his foolishness instantly–and a Proctor is always marked for calumny therefore.”
A calumny is a false statement made specifically for the reason of hurting someone's reputation. Miller is trying to point out two things: both the strength of Proctor's character and the susceptibility of the town to fear, envy and paranoia. Proctor is a man who know his own mind. He does not fear the attitudes of others and he refuses to do things just to "look good." The best example of this is Proctor's refusal to attend church, not because he lacks faith, but because he recognizes the hypocrisy of Parris. Therefore, because Proctor is so strong in his own beliefs, those around him quickly feel foolish for giving into social pressure and hysteria. The natural result of these "fools" is to criticize and impune Proctor, in order to make Proctor look worse and for the speaker to look better.
In other words, calumnies are the work of insecure bullies.