I wrote unto you in an epistle not to company with fornicators:  Yet not altogether with the fornicators of this world, or with the covetous, or extortioners, or with idolaters; for then must ye needs go out of the world.  But now I have written unto you not to keep company, if any man that is called a brother be a fornicator, or covetous, or an idolater, or a railer, or a drunkard, or an extortioner; with such an one no not to eat.  For what have I to do to judge them also that are without? Do not ye judge them that are within?  But them that are without God judgeth. Therefore put away from among yourselves that wicked person.
1 Corinthians 5:9-13

Be sure to read all of chapter five.