Gemara

Question (Mar Ukva): There are people that torment me. I could inform on them to the king. Should I?

Answer (R. Elazar): "I will guard my ways from sinning with my tongue, I will muzzle my mouth while the evil one is in front of me" - even while the evil one is in front of me, I will muzzle my mouth.

Mar Ukva: They are bothering me excessively. I cannot bear them!

R. Elazar: "Be silent to Hash-m and Hischolel (hope) to Him" - wait for Hash-m, and He will make your enemies fall Chalalim Chalalim (piles of corpses). Rise early to the Beis Medrash and stay late, and your enemies will perish by themselves.

Just after R. Elazar said this, Geneiva (who was bothering Mar Ukva) was put in chains to be executed.

Rosh Hashanah 17a: Heretics, Mosrim... and people who intimidate the Tzibur descend to Gehinom and are judged there for all generations... Gehinom finishes and they do not.

Rishonim

Rif (Bava Kama 43b): We learn from Gitin that even if one is pained, he may not be Moser (hand over) a Yisrael or his property to the Nochri kingdom.

Rambam (Hilchos Chovel u'Mazik 8:9): One may not be Moser a Yisrael or his property to Nochrim, even if he is a Rasha and afflicts and pains him. Anyone who is Moser a Yisrael or his property has no share in the world to come.

Rambam (11): If one afflicts and pains the Tzibur, one may be Moser him to Nochrim to beat, incarcerate and fine him. One may not be Moser for paining an individual.

Chasam Sofer (7a DH Bnei): Even though Geneiva was merely reviling Mar Ukva, had there not been a solution to come early and stay late, it would have been permitted to be Moser him. Rashi connotes that had he been harassing him monetarily and through fraud, it would be permitted to be Moser him. Also the Rambam forbids Mesirah due to pain, implying that if he causes losses, one may be Moser to save his money. It seems that the Rambam learned from here to permit Mesirah of one who pains the Tzibur. The solution of coming early and staying late does not apply to a Tzibur. Who will do so on behalf of all of them? If one's tormentor causes Bitul Torah and Tefilah, since he cannot come early and stay late, everything is permitted

Pnei Yehoshua (Gitin 7a DH Gemara): R. Elazar first told Mar Ukva that he may not even be Moser Din l'Shamayim, and all the more so not to be Moser to the kingdom. When he heard that Mar Ukva cannot endure it, he counseled to come early and stay late. Tosfos (7a DH Haskem) explains that he permitted him to pray for the downfall of his tormentor. Because he had no recourse through Beis Din, it was permitted to be Moser Din l'Shamayim. Alternatively, he told him to come early and stay late in the Beis Medrash, and in this merit his enemy would perish by himself.

Ben Yehoyada (Gitin 7a DH Benei and DH Haskem): Geneiva was causing bodily pain to Mar Ukva. Nevertheless, R. Elazar advised pardoning it. Mar Ukva then said that he was concerned lest Geneiva be Moser him, so R. Elazar counseled how to protect himself. By being early and late in the Beis Medrash your enemies will perish by themselves, i.e. they will stumble in their own evil. It will not be considered that they are punished due to you, so you will be clear.

Poskim

Shulchan Aruch (CM 388:9): One may not be Moser (hand over) a Yisrael or his property to Nochrim, even if he is a Rasha and afflicts and pains him.

Rema: This is if he merely pains him verbally. If Levi is Moser David, David may be Moser Levi. He may even kill him, if there is concern lest Levi be Moser again, unless he could stop him in another way.

Beis Yosef (DH u'Mah she'Chosav Afilu): The Rosh (Teshuvah 17:2) says that one may be Moser a Moser, or even directly kill him, like Rav Kahana did (Bava Kama 117a). Geneiva was not a Moser. He was great in Torah, just he was bothering Mar Ukva. The Rashba (181) agrees.

Darchei Moshe (11*): The Rambam disagrees. He does not permit to be Moser one who pains an individual. However, perhaps he permits to be Moser one who is Moser an individual.

Rema: If he could stop him in another way, but was Moser him, this is like two who were Moser each other. Whoever damaged more pays the full excess of the damage.

Mordechai (Bava Kama 195): The Heter to kill or be Moser a Moser is only to stop him from damaging. If not, one who kills him or damages his money is liable to die or pay. We exempt him only from the fine of excommunication, for he merely reacted out of anger.

Shulchan Aruch (ibid.): Anyone who is Moser a Yisrael or his property has no share in the world to come.

Shach (53): His slaughter is Pasul like that of an idolater. If he wrote a Sefer Torah, it is Pasul.

Shulchan Aruch (12): If one is Moser the Tzibur and pains them, one may be Moser him to Nochrim to beat, incarcerate and fine him.

Shach (59) and Gra (71): The text should say 'if one is Meitzar (afflicts) the Tzibur...', like it says in the Tur and Rambam.

Source #1 (Bach DH u'Mah): Yevamos 90b says that Beis Din can lash and punish not according to Torah, to make a fence for Torah, when people rampantly transgress. They saw fit to do so for one who pains the Tzibur, but not for one who pains an individual.

Source #2 (Gra 72): Rosh Hashanah 17a equates Mosrim and people who intimidate the Tzibur.

Shulchan Aruch (ibid.): One may not be Moser for paining an individual.

SMA (30): If he was Moser money, and all the more so if Yisro (he afflicted) lashes and bodily punishments, it is permitted, like the Rema above (Sa'if 9).

Note: What is the Kal va'Chomer? Mesirah of money can lead to death! The Rema above discussed only Mesirah. It seems that the text of the SMA should say 'Mosro' (was Moser for) in place of 'Yisro.' (Also, the words 'lashes and bodily punishments' connote that authorities beat the victim.)

Shach (60): One may not be Moser due to pain, but if he regularly hits, one may be Moser him to save himself. We do not distinguish between Moser an individual and Moser a Tzibur. However, the Rambam says that if one was already Moser, we do not kill him unless he is Muchzak to do so. Perhaps regarding a Tzibur, we may be Moser him even if he is not Muchzak.