Gemara

Ezra decreed that a Ba'al Keri (one who had a seminal emission) may not say Divrei Torah (or pray) until he immerses.

Berachos 22a (Rav Nachman bar Yitzchak): Nowadays, people follow the lenient opinion of R. Yehudah ben Beseira:

Beraisa - R. Yehudah ben Beseira: (A Ba'al Keri may learn Torah because) Divrei Torah are not Mekabel Tum'ah.

Some say that Ze'iri came and taught that Chachamim abolished the immersion.

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

Contradiction (Beili citing Rav): 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: Daniel decreed in the city; Beis Hillel and Beis Shamai decreed even in the field.

Question #1 (Shmuel): R. Simla'i taught that R. Yehudah Nesi'ah and his Beis Din voted and permitted it, because Nosen Ta'am li'Fgam is permitted

(Mishnah): A Beis Din cannot nullify the enactment of another Beis Din, unless it is bigger in Chachmah and Minyan. (R. Yehudah Nesi'ah was not bigger in Chachmah and Minyan than Daniel, Beis Hillel and Beis Shamai)!

Question #2: Rabah bar bar Chanah taught that no Beis Din, even Eliyahu ha'Navi's (in the future), can nullify one of the 18 decrees!

Answer (Rav Mesharshiya): The restrictions apply to decrees that spread through most of Yisrael. The decree against oil did not spread (therefore even a smaller Beis Din can Mevatel, and even though it was among the 18 decrees).

(R. Shimon ben Gamliel and R. Eliezer): We make a decree only if the majority of Yisrael can fulfill it.

Rav Ada bar Ahavah: "Ba'Me'erah Atem Ne'arim... ha'Goy Kulo" - a decree is proper only if the entire nation can fulfill it.

37a: R. Yehudah Nesi'ah: Last night, we permitted oil of Nochrim.

R. Simla'i: You will even permit their bread!

R. Yehudah Nesi'ah: No, for then we would be called a permissive Beis Din.

Rishonim

Rif and Rosh (Berachos 13b and 3:21): Some say that they abolished the Tevilah both for prayer and for Divrei Torah. Some say that this was only for Torah, but one must immerse for prayer, or wash with nine Kavim. Rav Hai Gaon says that since the Gemara omits this (regarding prayer), we follow the custom of Yisrael: a Ba'al Keri does not pray until washing, even if he lacks water.

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. They relied on R. Shimon ben Gamliel and R. Eliezer, who say that we decree only if the majority of Yisrael can fulfill it.

Ran (Avodah Zarah 14b DH Rebbi): No Beis Din, even Eliyahu ha'Navi's (in the future), can nullify one of the 18 decrees! R. Yehudah Nesi'ah and his Beis Din were able to permit oil of Nochrim, even though it was among the 18, because the decree did not spread. They relied on R. Shimon ben Gamliel. I.e. even though the decree did not spread, we still require a greater Beis Din. However, R. Shimon ben Gamliel taught that we decree only if the majority of Yisrael can fulfill it. There are three laws. If a decree spread throughout Yisrael, no Beis Din, even Eliyahu's (in the future), can nullify it. This is the reason why we cannot permit the (other 17 of the) 18 decrees. If the decree did not spread, but the majority of Yisrael could fulfill it, a greater Beis Din is required. If the decree did not spread because the majority of Yisrael cannot fulfill it, even a smaller Beis Din can Mevatel it. A Heter is required. R. Yehudah Nesi'ah would have permitted bread of Nochrim, if not that he would be called a permissive Beis Din. We must say that the Tzibur could not fulfill the decree, yet a Heter was required. This is why the Ramban said that even nowadays, if Gedolei Yisrael would agree, we could permit Nochri bread. Even though we are smaller, also R. Yehudah Nesi'ah's Beis Din was smaller than the Talmidim of Hillel and Shamai.

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.

Rambam (Hilchos Kri'as Shma 4:8): Ezra and his Beis Din enacted that a Ba'al Keri not read Divrei Torah until he immerses, unlike other Temei'im. The enactment did not spread throughout Yisrael, and most of the Tzibur could not fulfill it. Therefore, it was Batel. It is the custom of all Yisrael to read Torah and Shema when they are Ba'al Keri, for Divrei Torah are not Mekabel Tum'ah.

Kesef Mishneh: The Rambam explains that a Beis Din did not Mevatel Ezra's enactment. If so, he would have said who abolished it. Also, why do we say that people conduct like R. Yehudah ben Beseirah? We should say that the decree was abolished! Also, we find late Amora'im who immersed for Keri, and distinguish when a sick person must immerse. Rather, 'they abolished it' means that they did not force people to fulfill it, and it became Batel by itself. If it was only because it did not spread, granted, another Beis Din could abolish it, but Bitul would be needed. Rather, since the Tzibur was unable to fulfill it, Chachamim did not force people. This is why the Rambam writes 'it was Batel', not 'they were Mevatel it.' This is why later Amora'im fulfilled it. They could not force people, but since they themselves were able to, they fulfilled it.

Minchas Yitzchak (3:15:12): A smaller Beis Din can Mevatel any enactment that did not spread. Why did the Rambam need to say that it did not spread because Beis Din did not enforce it? The Rambam derived from the Gemara that there was no explicit Bitul, because it did not spread. Not spreading allows Bitul; since the Tzibur could not fulfill it, it was automatically Batel.

Tosfos (Avodah Zarah 36b DH Iy): Even an enactment that did not spread is in force until a Beis Din is Mevatel it. R. Yehudah needed to permit oil.

Rambam (Hilchos Tefilah 4:5,6): This enactment was Batel, because it did not spread throughout Yisrael, and most of the Tzibur could not fulfill it. The custom in Sefard and Bavel is that a Ba'al Keri does not pray until he bathes his entire body in water, due to "prepared to greet Elokecha." However, there is no custom to bathe for one who was sick or saw Keri due to Ones.

Kesef Mishneh: The enactment was Batel only regarding Divrei Torah. Regarding prayer, even though we do not require Tevilah, one must bathe in nine Kavim of water, like the Rif says.

R. Mano'ach: The enactment was Batel by itself because most of the Tzibur could not fulfill it. They did not vote to annul it, for this would require a Beis Din greater in number and Chachmah. In our lands the custom is only that everyone immerses on Erev Yom Kipur. It is a mere custom, therefore one does not bless on it.

Hagahos Maimoniyos (5): The custom of Yisrael is not to pray until bathing, even if they don't have water. R. Yehudah ben Beseirah was lenient only for Divrei Torah. One who is stringent will live a long life.

Rambam (Introduction to Mishnayos Zera'im 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". One example is Ezra's enactments.

Poskim

Shulchan Aruch (OC 88:1): Any Tamei person may learn Torah, read Shma or pray, except for a Ba'al Keri. Ezra forbade this so Chachamim would not be too frequent with their wives. Later, the decree was abolished; he is permitted without Tevilah or having nine Kavim poured over him. This is the practice.

Mishnah Berurah (3): The decree that a Ba'al Keri may not learn Torah or pray was annulled because most of Yisrael could not fulfill it. It entailed Bitul Torah and Bitul Peru u'Rvu.

Eshel Avraham (1): The Kesef Mishneh explains that a Beis Din did not Mevatel Ezra's enactment. Therefore, one who fulfills it will be blessed.

Be'er Heitev (1): The Rambam said that he himself was never Mevatel Takanas Ezra (brought in R. Yonah 13b Sof DH Ki).