If any man doesn't obey our word in this letter, note that man, that you have no company with him, to the end that he may be ashamed.
Paul put social pressure to good use here. It is regrettable that in our day social pressure often has very little influence on erring brethren. Rather than submit to church discipline many Christians simply change churches. Strong measures may be necessary in some cases so the offender feels the need to repent and to live in harmony with the will of God.