Adam Clarke on Titus 3:10

A man that is a heretic - Generally defined, one that is obstinately attached to an opinion contrary to the peace and comfort of society, and will neither submit to Scripture nor reason. Here it means a person who maintains Judaism in opposition to Christianity, or who insists on the necessity of circumcision, etc., in order to be saved. This is obviously the meaning of the word heretic in the only place in which it occurs in the sacred writings.

After the first and second admonition, reject - Labour to convince him of his error; but if he will not receive instruction, if he have shut his heart against conviction, then - burn him alive? No: even if demonstrably a heretic in any one sense of that word, and a disturber of the peace of the Church, God gives no man any other authority over him but to shun him, παραιτου. Do him no harm in body, soul, character, or substance; hold no communion with him; but leave him to God. See the notes on Act 5:17; Act 24:14 (note), where the word heresy is particularly explained.