Gemara

Nedarim 50b: A woman came before Rav Yehudah for judgment. The outcome was against her. She asked 'would your Rebbi, Shmuel, have judged thusly?'

Rav Yehudah: Did you know him?

The woman: Yes. He was short, dark, and had a large stomach and big teeth.

Rav Yehudah excommunicated her for disgracing him. She died.

Mo'ed Katan 16a (Rav Chisda): If someone disgraces a Chacham, we excommunicate him immediately, without warning.

16b: A woman was sitting with her leg stuck out, sifting barley. A Chacham passed by. She did not show deference. The Chacham told Rav Nachman.

Rav Nachman told her 'if you did not hear the Talmid excommunicate you, considered yourself in Nezifah (banished) for one day.'

Yevamos 105b: R. Yishmael bar R. Yosi walked slowly to his place in Rebbi's Beis Medrash. Avdan (a Talmid) rebuked him for (seemingly) stepping on the heads of Am Kadosh, and suggested that he is not fitting to learn from Rebbi.

R. Yishmael: Was Moshe fitting to learn from Hash-m?

Avdan: Are you Moshe?!

R. Yishmael: Is your Rebbi Hash-m?!

(Rav Yosef): Rebbi (did not object to Avdan's treatment of R. Yishmael, and) received his punishment. He was called your Rebbi, i.e. but not my Rebbi.

A Yevamah came. Rebbi told Avdan to check whether or not she is an adult. Avdan left. R. Yishmael said that his father allows a Ketanah to do Chalitzah. Rebbi accepted this, and said that there is no need to check her. When Avdan was returning to his place, R. Yishmael said 'only one needed by Am Kadosh may step on the heads of Am Kadosh!' Rebbi told Avdan to stay in his place.

(Beraisa): Just then, Avdan became a Metzora, his two sons drowned, and his two daughters-in-law did Mi'un.

Rav Nachman bar Yitzchak: Blessed is Hash-m, Who embarrassed Avdan in this world (to spare him shame in the world to come).

Berachos 19a (R. Yehoshua ben Levi): Beis Din excommunicates for the honor of Chachamim (or their enactments) in 24 cases in Mishnayos.

R. Elazar found three, i.e. disgracing Netilas Yadayim, denigrating a deceased Chacham, and acting haughtily towards Hash-m.

(Mishnah): Akavya ben Mahalal'el used to say that a convert or freed slave may not drink Sotah water (the Parashah applies only to "Benei Yisrael");

Chachamim: Shemayah and Avtalyon once ordered that a freed slave drink Sotah water!

Akavya: They gave Dugma (a similar matter) to drink.

Chachamim excommunicated Akavya for saying this, and stoned his coffin.

R. Yehudah: Heaven forbid! There was no one as great in wisdom, purity, and fear of sin as Akavya! Rather, they excommunicated Elazar ben Chanoch for disgracing Netilas Yadayim.

(Mishnah): (Bnei Yisrael asked Choni ha'Me'agel to pray for rain. Choni gave Hash-m an ultimatum, and was not satisfied until Hash-m sent the exact kind of rain he desired.)

Shimon ben Shetach (to Choni): You deserve Niduy. If you were not Choni, I would excommunicate you! However, I see that you whine in front of Hash-m, and he does your will like a father does for a son!

R. Yehoshua extrapolates (any lack of respect for Chachamim is worthy of Niduy). R. Elazar found only three places, for he does not extrapolate.

Rishonim

Rif (Bava Kama 91a): Reish Lakish fined a man a Litra of gold for being upset with R. Yehudah bar Chanina (Yerushalmi 8:6).

Rambam (Hilchos Talmud Torah 6:12): If witnesses testify that Levi disgraced a Chacham, even verbally, Beis Din publicly excommunicates him.

Hagahos Maimoniyos (7): This is only if he disgraced him. The Yerushalmi holds that we do not excommunicate for a lack of honor, e.g. not standing up.

Rebuttal (Panim Me'iros 2:168, cited by Pischei Teshuvah YD 243:6): Mo'ed Katan 16b teaches otherwise. Others explain the Yerushalmi differently.

Rambam (ibid.): In every place we also fine him to pay a Litra (a weight) of gold to the Chacham.

Beis Yosef (YD 334 DH Kosav ha'Rambam): This includes Bavel. However, nowadays we do not collect this fine.

Teshuvas Rosh (15:10, cited in Bedek ha'Bayis YD 243 DH Chosav ha'Rosh): We fine a Litra of gold for anyone whose Torah is primary and his work is haphazard. I.e. he fixes time for Torah and never skips them, and learns whenever he is not engaged in earning money necessary for his household.

Rambam (14): A man or woman is excommunicated for any of the following 24 matters: disgracing a live or dead Chacham...

Tosfos (Kidushin 28a DH ha'Korei): Wherever the Gemara says that one is excommunicated for something, this is not automatic. One who sees must put the person in Cherem.

Rambam (7:13): A Chacham may excommunicate for his honor, but it is not fitting to do so. Rather, he should ignore what Amei ha'Aretz say if he was disgraced in private. If it was in public, he may not pardon his honor. If he does, he will be punished. He must avenge this like a snake, until the offender asks forgiveness and he pardons him.

Kesef Mishneh: The Rivash (220) says that a Chacham can pardon his honor regarding matters like standing for him, but he cannot pardon disgrace.

Teshuvas Rambam (Blau Sof 268): If an appointed Rav rebukes a smaller Chacham about an Isur, and the smaller Chacham persists, he disgraces the Rav and Hash-m's word. He should be excommunicated.

Ra'avad (Eduyos 5:6 DH Dugma): Akavya said that they gave Dugma to drink. I.e., they were also (descended from) converts, and wanted to show that the laws of Yisrael apply also to converts (but really, she may not drink). Some say that it means that they gave her imitation Sotah water, to scare her into admitting.

Tosfos Yom Tov (DH v'Niduhu and DH Chas): The latter Perush holds that Akavya's sin was suspecting them of erring. Giving false water could make people say that the water does not really check women! R. Yehudah holds that Akavya did not mean that they intended to give to her to drink. They just wanted to scare her into admitting.

Poskim

Shulchan Aruch (YD 243:7): If witnesses testify that Levi disgraced a Chacham, even verbally (Rema - even not in front of him), Beis Din publicly excommunicates him.

Erech Lechem: The Rivash (220) says that we excommunicate even for not being submissive to a Chacham. Some disagree.

Gilyon Maharsha: This is even if he did not intend (to disgrace him), in any case in which he cannot explain why he did what he did.

Rema: Nowadays we do not have a Chacham for whom we fine a Litra of gold for disgracing him. We do fine according to the one who embarrassed and the one who was embarrassed.

Rashba (2:291, cited in Beis Yosef DH v'Chosav ha'Rashba): We fine for one who is respected by the Tzibur for his Torah. One who disgraces him disgraces Torah. One pays more for disgracing him, since everyone else honors him. However, if he does not deal honestly and pleasantly with people, he disgraces Torah and there is no Mitzvah to honor him.

Shulchan Aruch (334:43): A man or woman is excommunicated for any of the following 24 matters: disgracing a live or dead Chacham...

Beis Yosef (DH v'Chosav): This is only if the Chacham was fit for Hora'ah and toiled in Torah.

Rivash (33, cited in Beis Yosef DH Kosav ha'Rivash): A Talmid can excommunicate for his own honor or the honor of his parent, all the more so for the honor of his Rebbi, who brings him to the world to come! Everyone must observe the stringencies of the Cherem, except for the Rebbi or someone greater than the Talmid in Chachmah and Yir'as Shamayim.