3 John 1:10

I wrote to the church, but Diotrephes, who loves to be first among them, does not receive us. Therefore, if I come, I will bring to light the works he is doing, gossiping wickedly about us. And not content with that, he not only does not receive the brothers himself, but also forbids those who want to receive them and expels them from the congregation.
"I will bring to light the works he is doing." If it is commanded that evil should not be repaid with evil (Rom. 12:17), what is it that he now threatens? Therefore, we say that this command is so that we do not repay evil to him who has sinned only against ourselves: For if it is a sin against us, and it brings an obstacle to faith, it should be referred to such, as Paul did to Elymas who was perverting the ways of the Lord. (Acts 13:10)
"gossiping wickedly." That is, throwing insults and cursing.

For this reason, if I come, I will bring up his deeds. That is, by openly reproving, I will bring his works to the knowledge of all. Just like the apostle Paul said: "What do you desire? Shall I come to you with a rod?"

He acts, talking against us with malicious words. It should be noted that we should not arouse the tongues of detractors by our fault, lest they perish, but we should equally bear those that are incited by their own malice, so that merit may increase for us; sometimes, however, we should also curb them, lest while they disseminate evil about us, they corrupt the hearts of the innocent who could have heard good things. Hence John reproved the tongue of his detractor, lest those who could have listened might not hear his preaching and remain in their wrong ways.

If it is forbidden to repay evil for evil (Rom. 12:17), then how is the present threat of the Apostle to be explained? We answer: the commandment speaks of not repaying evil for evil to the one who sins against us, but when sin against us also causes harm to the faith, then the one committing such sin must be punished, just as Paul also brought evil upon Elymas, who was turning people away from the paths of the Lord (Acts 13:8–11).