LEV601 [This verse]. However, when the matter is clear to all – – it is known, tested, and investigated--that the sinner hates rebuke and will not listen to the voice of his instructors, nor turn an ear to his teachers, concerning this the pasuk says (Mishlei 9:8), "Do not reprove a scoffer, lest he hate you." Our Sages said (Yevamos 65b), "Just as it is a mitzvah to say what is listened to, so it is a mitzvah to not say what is not listened to," (Beitzah 30a), "Better that they be unintentional [sinners], than willful ones" (By rebuking him, he will only cause him to sin more. See Shulchan Aruch, Orach Chaim 608:2 for the conditions to this exemption).