Gemara

Megilah 2a - Question: If Anshei Keneses ha'Gedolah decreed that all must read on the 14th or 15th, how could later Chachamim uproot this?

Mishnah: A Beis Din can overturn the decree of a previous Beis Din only if it is greater in Chachmah and Minyan (number of Talmid, or number of Chachamim who agreed to the decree)!

Answer: Anshei Keneses ha'Gedolah themselves enacted all these days.

Mo'ed Katan 3b - R. Shimon ben Pazi: R. Gamliel and his Beis Din permitted these times (of Tosefes Shemitah).

Question (R. Zeira): How could they overturn the enactment of Beis Shamai and Beis Hillel? To overturn, a Beis Din must be greater in Chachmah and Minyan!

Answer (R. Avahu): They stipulated from the beginning that if a future Beis Din wants, they may overturn it.

Gitin 36b - Question: Did Hillel enact Pruzbul for his generation, or for all generations?

If he only enacted for his generation, any later Beis Din can abolish it. If he enacted for all generations, a Beis Din can abolish the enactment of another Beis Din only if they are greater in Chachmah and Minyan.

Answer: Shmuel said 'Pruzbul is Ulbana (audacity) of the judges. If I had the power, I would abolish it.' (Surely, Shmuel was smaller in Chachmah than Hillel!)

Rejection: Shmuel meant 'if I was greater than Hillel I would abolish it.'

Avodah Zarah 36a (Rav): Daniel decreed against oil of Nochrim.

Question: Rav himself taught that the prohibitions of Nochri bread, oil, wine and (relations with) Nochri girls are among the 18 decrees (made on the day of heated fighting between Beis Shamai and Beis Hillel)!

Answer: Rather, Daniel decreed in the city, and Beis Shamai and Beis Hillel decreed even in the field.

Question #1: R. Yehudah Nesi'ah and his Beis Din voted and permitted it, because Nosen Ta'am li'Fgam (the oil absorbs Isur from the Nochri's Kelim) is permitted.

Mishnah: A Beis Din can overturn the decree of a previous Beis Din only if it is greater in Chachmah and Minyan. (R. Yehudah Nesi'ah was not greater!)

Question #2: Rabah bar bar Chanah taught that a (greater) Beis Din can nullify the decree of another Beis Din, except for the 18 decrees. Even Eliyahu and his Beis Din cannot nullify them!

Answer to both questions - Rav Mesharshiya: The restrictions apply to decrees that spread through most of Yisrael. The decree against oil did not spread.

Rishonim

Rif and Rosh (Avodah Zarah 14b and 2:27): R. Yehudah ha'Nasi and his Beis Din permitted oil of Nochrim. Even though it was among the 18 decrees, since the decree did not spread, they were able to Mevatel it.

Rambam (Hilchos Mamrim 2:2): If Beis Din made a decree or enactment and it spread throughout Yisrael, a later Beis Din cannot nullify it unless they are bigger in Chachmah and Minyan.

Rebuttal (Ra'avad): If it spread throughout Yisrael, even Eliyahu and his Beis Din cannot nullify it, like it says in Avodah Zarah (36a).

Kesef Mishneh: The Rambam holds that a later Beis Din can alter a decree or enactment if they are greater, but even a greater Beis Din cannot abolish a fence.

Lechem Mishneh: The Rambam holds that all fences to prevent transgression are like the 18 decrees; they can be annulled only if they did not spread. Tosfos (Avodah Zarah 36a DH veha'Tenan) says that the 18 are more stringent because they were decreed amidst mortal fighting, but other decrees can be annulled by a bigger Beis Din.

Question: Shmuel said that if he was greater than Hillel he would abolish Pruzbul (which was a fence lest people transgress refusing to lend)!

Answer (Kesef Mishneh): That was not a fence for an active Isur, for refusing to lend is inaction. Also, the enactment was primarily to help the poor (obtain loans) and the rich (get back their money), like Rav Chisda explained, 'Pruz Buli and Buti'. Perhaps this question was his source to say that it was not a fence lest people transgress!

Question (Kesef Mishneh): The Gemara asks, since R. Yehudah Nesi'ah was not greater than Beis Shamai and Beis Hillel, how could he permit Nochri oil? This was a decree to prevent Isurim (lest Yisraelim drink Nochri wine and marry Nochriyos and serve idolatry...). Even a greater Beis Din may not Mevatel it!

Answer #1 (Kesef Mishneh): Indeed, the Gemara asks that even if it would have been a decree which a bigger Beis Din may annul, R. Yehudah Nesi'ah was smaller!

Question: The Ra'avad holds that any decree that spread cannot be annulled even by a bigger Beis Din; if it did not spread, even a smaller Beis Din can annul it. How does he explain Pruzbul (Shmuel would have abolished it if he was greater than Hillel)!

Answer (Lechem Mishneh): The Ra'avad holds that if an enactment did not spread and the Tzibur cannot fulfill it, even a smaller Beis Din can annul it. If it did not spread but the Tzibur could fulfill it, only a greater Beis Din can annul it.

Question: The Gemara (Mo'ed Katan 3b) asked how R. Gamliel permitted Tosefes Shemitah, because his Beis Din was not greater. Tosefes Shemitah is a fence lest people transgress Shemitah, so no Beis Din can overturn it!

Answer (Lechem Mishneh): Beis Din was not concerned lest people transgress Shemitah. It was merely better that people refrain before Shemitah.

Rambam (ibid.): This is even if the reason for the decree or enactment no longer applies.

Source (Kesef Mishneh): If something was forbidden through a Minyan, another Minyan is needed to abolish it (Beitzah 5a). This refers to a bigger Minyan. Greater Chachmah is also required; the Gemara merely listed one of the requirements.

Question (Ra'avad): There was an enactment (Beitzah 5a) not to redeem Neta Revai near Yerushalayim, and R. Yochanan ben Zakai annulled it because the reason no longer applied, even though he was smaller!

Answer #1 (Kesef Mishneh): Perhaps he was bigger. The Gemara says that he was the smallest of Hillel's Talmidim, but perhaps that refers to other Chachmos. It says that he knew everything in Torah, including Ma'aseh Merkavah. Surely if he was the smallest in Chachmah he would not have been appointed Nasi! Also, even if he was the smallest, it is possible that he was greater than those who made the enactment. We find that sometimes later Chachamim surpassed previous Chachamim in Chachmah, e.g. Rava was greater than Rav Yehudah (Ta'anis 24a).

Answer #2 (Lechem Mishneh): Ma'aser Sheni (5:2) says that when there were too many fruits they enacted that it may be redeemed near the wall.

Note: Perhaps a third answer is suggested by the distinction the Rambam makes between different kinds of decrees (see below). He mentions the stringency not to annul decrees only regarding fences to prevent transgression of Torah.

Rambam (Introduction to Mishnayos Zera'im DH ha'Chelek ha'Revi'i): Nevi'im and Chachamim enacted decrees in every generation to be a fence for Torah. The Torah commands "U'Shmartem Es Mishmarti" - make a protection for My protection (over you, i.e. the Torah - Yevamos 21a). The Isur of fowl with milk is an example. If such a decree spread, even Nevi'im may not oppose it. The Gemara says that even Eliyahu and his Beis Din could not Mevatel the 18 decrees because they spread throughout Yisrael.

Rambam (ibid. DH ha'Chelek ha'Chamishi): Some enactments do not add or detract from Mitzvos. Rather, they are helpful for Torah. If one transgresses them "Poretz Geder Yishchenu Nachash". Examples are the enactments of Moshe, Yehoshua, Ezra, R. Gamliel, R. Yochanan ben Zakai, Chachamim of Usha....

Question: The Gemara (Beitzah 4b) asked why we keep two days of Yom Tov after the calendar was fixed (in the days of the Amora'im). It answered that we guard the custom of our fathers, lest there be decrees against Torah and people will miscalculate. If not, they would have abolished Yom Tov Sheni, even though the Amora'im were smaller Chachamim than those who made the enactment!

Answer (Kesef Mishneh): The question was why those who enacted two days of Yom Tov did not rather enact to fix the calendar. Alternatively, it was known that those who enacted it stipulated that if the calendar will be fixed one day of Yom Tov suffices. According to the Ra'avad, the Gemara should have tried to answer that a smaller Beis Din cannot Mevatel, and rejected this because the reason no longer applies.