what convinces cunningham to tell other men to leave ?

Scout's conversation with Mr. Cunningham emphasizes her knowledge of young Walter Cunningham and reminds Mr. Cunningham of the human bonds that connect everyone in the town. From the indistinguishable group of men, she singles him out and restores his individuality out of anonymity by addressing him by name and recalling his son and entailment. When people join together in a mob, they lose a feeling of responsibility for their actions, because they act as a group rather than as separate individuals. Scout's ability to separate Mr. Cunningham from his group is a result of the sheer innocence of her statements, which shows how inconceivable violence is to her, and forces them to reconsider their behavior. Mr. Cunningham, confronted with the shame of the group's plans and having been reminded of his own responsibility in them, decides to remove himself from the scene, and everyone else follows.