Read Matthew 18:15-18 (NIV)
15"If your brother sins against you, go and show him his fault, just between the two of you. If he listens to you, you have won your brother over. 16But if he will not listen, take one or two others along, so that 'every matter may be established by the testimony of two or three witnesses.' 17If he refuses to listen to them, tell it to the church; and if he refuses to listen even to the church, treat him as you would...