Here’s how St. Augustine, as a bishop, applied that verse from St. Paul:
[quote=St. Auguistine]1. The Apostle Paul has said: A man that is an heretic after the first and second admonition reject, knowing that he that is such is subverted and sins, being condemned of himself. Titus 3:10-11 But though the doctrine which men hold be false and perverse, if they do not maintain it with passionate obstinacy, especially when they have not devised it by the rashness of their own presumption, but have accepted it from parents who had been misguided and had fallen into error, and if they are with anxiety seeking the truth, and are prepared to be set right when they have found it, such men are not to be counted heretics. Were it not that I believe you to be such, perhaps I would not write to you.And yet even in the case of a heretic, however puffed up with odious conceit, and insane through the obstinacy of his wicked resistance to truth, ** although we warn others to avoid him, so that he may not deceive the weak and inexperienced, we do not refuse to strive by every means in our power for his correction.**
So in other words, St. Paul’s admonition is not absolute. It is intended to protect us from having our own faith corrupted.