A man should be removed from society if he admits he is not virtuous, because virtue is a requirement for men to live together.
Protagoras is saying that society contains some men who have no potentiality for virtue and, therefore, have no place in society. The great majority, however, have the ability to learn justice and respect for others if they are willing to be taught. We do not ridicule men who are deformed because they cannot help their condition, but men with vices are admonished when they operate contrary to civic virtue. Men are punished not because of the crime they committed but to prevent the next crime --punishment teaches virtue.