Matthew 18:15-17

15 "If a believer does something wrong, go, confront him when the two of you are alone. If he listens to you, you have won back that believer.
16 But if he does not listen, take one or two others with you so that every accusation may be verified by two or three witnesses.
17 If he ignores these witnesses, tell it to the community of believers. If he also ignores the community, deal with him as you would a heathen or a tax collector.