Gemara

Kidushin 20b (Tana d'Vei R. Yishmael) Suggestion: If one sold himself to be a servant of idolatry, perhaps (we abandon him and) he is not redeemed!

Rejection: "There will be redemption for him."

72a: When Rebbi was dying, he revealed certain cities in Bavel with lineage problems. One was full of converts from Amon. Another was full of Mamzerim. One has brothers who swap wives (which makes Mamzerim);

He saw (through Ru'ach ha'Kodesh) that R. Acha bar Yoshiyah excommunicated residents of a certain city for transgressing Shabbos, and they became heretics.

Sotah 47a (Beraisa): One should dispel with the left hand, and bring close with the right, unlike Elisha and Yehoshua ben Perachyah, who utterly dispelled.

Gechazi took money from Na'aman. Elisha cursed him "Na'aman's Tzara'as will cling to you forever." Later, Elisha went to get Gechazi to repent, but failed.

An innkeeper showed great honor to Yehoshua ben Perachyah. He said 'our hostess is very nice.' A Talmid (Yeshu) said 'but her eyes are round'!

Yehoshua called him a Rasha for gazing at married women. He excommunicated him, with 400 people to publicize it. Each day, the Talmid came before him, but he refused to see him. Later, Yehoshua went to get him to repent, but failed.

Bava Kama 69a (Mishnah): People used to mark off Revai (fourth-year Peros) with clods off earth, and Orlah (Peros in the first three years) with pottery shards.

R. Shimon ben Gamliel says, this is only in Shemitah, when people may enter others' fields. In other years, one who enters another's field is a thief. We do not care if the food he steals is forbidden!

The Tzenu'im would take coins and say 'whatever (Revai) was gathered from my field is redeemed on these coins.'

Beitzah 30a: Mid'Oraisa one must add to the fast of Yom Kipur. Nevertheless, we do not protest against those who eat and drink until the last moment. It is better that Yisrael transgress b'Shogeg, and not b'Mezid.

Rishonim

Terumas ha'Deshen (Pesakim 138): We force one to divorce his wife if she is repulsed by him, lest she go to Tarbus Ra'ah (a sinful lifestyle). However, we are not lenient in order to prevent someone from going to Tarbus Ra'ah. Why did the Gemara record Rebbi's prophecy that R. Acha excommunicated people who desecrated Shabbos, and that they became heretics? And why did Rebbi say this? This teaches that we excommunicate one worthy of this, even if it will cause him to go to Tarbus Ra'ah. We punish for past sins in spite of such concerns, all the more so we do not distort a ruling for the future! It is no transgression to force someone to divorce his wife. We know that he wants to obey Chachamim.

Tashbatz (2:46): It is better that people be Shogeg than Mezid even regarding something mid'Oraisa, if it is not explicit in the Torah. This is only when we are sure that they will not heed us. If not, we must protest. If we would allow people to sin b'Shogeg, the Torah would fall bit by bit. Also, this is only if everyone is Shogeg. If only a minority is Shogeg, we protest. This is even if they will come to be Mezid, for people learn from the improper, and not from the proper.

Poskim

Rema (YD 334:1): We excommunicate someone even if there is concern lest this cause him to go to Tarbus Ra'ah.

Rebuttal (Taz 1): How can we cause one to go to Tarbus Ra'ah? The Terumas ha'Deshen's proof from Rebbi's prophecy is invalid. Rebbi was warning about places in Bavel with lineage problems. He also warned people to distance from Resha'im of that city. Even though this was evident to people there, he wanted to prove that he had a prophecy, to prove that also what he said about lineage was true. Even after one became a servant of idolatry, we redeem him. All the more so, if one did not yet do idolatry, we may not cause him to do so! Mahari Mintz (5) permitted a man to marry a woman nursing another man's baby, to save them from going to Tarbus Ra'ah. (Chachamim forbade such a marriage, lest she become pregnant and wean the baby prematurely, and he will die.)

Rebuttal (Nekudas ha'Kesef): We can dispel all the Taz' proofs. We put one in Niduy if he deserves it. If not, judgment would be Batel from Yisrael!

Rema (48): Even though one must protest against transgressors, and one who does not protest is punished for their sin, one need not spend money for this. Therefore, the custom is to be lenient about protesting against transgressors when there is concern lest they harm our bodies or money.

Rema (CM 12:1): Some say that nowadays we do not protest against sinners, for it is dangerous, lest they inform to the king.

Chavos Ya'ir (141): A case occurred in which Ploni transgressed Yayin Nesech (wine offered to idolatry). His congregation wanted to fine him and announce about him. The Rav stopped them, lest he sin more. It seems that he is supported by the Rambam, Shulchan Aruch (CM 17:3, which allows witnesses to sit nowadays) and the episode with R. Yehoshua ben Prachyah. It is better for a Chaver to transgress a minor matter, so that an Am ha'Aretz will not transgress a severe matter (Eruvin 32a). It is better that people be Shogeg than Mezid about something not explicit in the Torah, even though there is a Mitzvas Aseh to rebuke. However, the Rav should have excommunicated Ploni. If we will be concerned lest people sin more, sinners will do what is straight in their eyes. We may not justify Resha'im, and cast truth and Hash-m's laws to the ground. We are concerned lest one sin greatly, only if he did not yet sin b'Mezid. It is better that people be Shogeg than Mezid when they would not accept rebuke, for then there is no Mitzvah of rebuke. The Rema is lenient about protesting against sinners when there is concern lest they harm our bodies or money, but not due to concern lest they sin more. R. Yehoshua ben Prachyah's error was to excommunicate his Talmid with great publicity. He was not worthy of Niduy. Only in Shemitah people used to mark off Revai and Orlah. In other years, in any case one who enters another's field sins; we are not concerned if his sin is compounded. The Sanhedrin was never concerned for this. Those who would find loopholes to exempt everyone from Misas Beis Din 'increase murderers in Yisrael' (Makos 7a). We were not concerned for the man who lost his case in Moshe's Beis Din and blasphemed. We deter converts, for they harm Klal Yisrael. This overrides the benefit that conversion would be for them! The Torah warns 24 times not to afflict a convert, lest he return to his former lifestyle, but we do not judge him leniently due to this concern. "There will be one law for native Yisre'elim and converts." Punishments in the Torah are primarily lest others be adversely influenced - "others will hear and fear." Beis Din may not pardon one who rebelled against their ruling, lest rebellion increase in Yisrael. Once, they killed one who rode on a horse on Shabbos, even though he did not deserve to die. It was needed for that generation.