Gemara

42a (Mishnah): If before Shemitah (the seventh year, in which whatever grows is Hefker) Shimon was Mudar Hana'ah from Reuven, Shimon may not enter Reuven's field, nor eat from trees whose foliage extends past Reuven's property;

(If he vowed) in Shemitah, he may not enter the field, but he may eat from what hangs outside.

(Rav and Shmuel): If Reuven told Shimon before Shemitah 'this property is forbidden to you', Shimon may not enter Reuven's field, nor eat from what hangs outside, even in Shemitah. If he vowed during Shemitah, Shimon may not enter the field, but he may eat from what hangs outside.

(R. Yochanan and Reish Lakish): If he said before Shemitah 'my property is forbidden to you', he may not enter his field, nor eat from what hangs outside. When Shemitah comes, he may not enter the field, but he may eat from what hangs outside.

R. Yochanan and Reish Lakish teach about when he says 'my property.' Rav and Shmuel teach about when he says 'this property'. They do not argue.

42b (Mishnah): In Shemitah he may not enter his field...

Question: He may eat from what hangs outside because it is Hefker (in Shemitah). For the same reason, he should be allowed to enter the land!

Answer #1 (Ula): The case is, the trees are on the border (one can take the fruits without entering the field).

Answer #2 (R. Shimon ben Elyakim): This is a decree, lest he tarry on the field longer than necessary to take the fruit.

Rishonim

The Rif brings our Mishnah and the opinions of Rav and Shmuel and R. Yochanan and Reish Lakish. All agree that one can forbid his property even after it leaves his Reshus/

Ritva (DH uva'Shevi'is): The Torah made the land Hefker in Shemitah in order to eat the produce. If not, it was futile to make the fruits Hefker! R. Shimon ben Elyakim says that we decreed not to enter the field, lest he tarry on the field after picking and eating the produce. The Torah did not make it Hefker for this. This is common, therefore Chachamim decreed to forbid even standing (i.e. to pick the fruits and leave immediately). The Halachah follows him.

Nimukei Yosef (DH Gezeirah): The Torah made the field Hefker only for picking the produce.

Ran (DH R. Shimon): We did not decree to forbid Bikur Cholim when the Choleh may not benefit from the visitor. Since people usually sit, it suffices to require the visitor to stand. This reminds him not to stay too long. We must decree regarding Shemitah, for there is no Heker to remind the picker to leave immediately.

Rambam (Hilchos Nedarim 6:13): If Reuven was Mudar Hana'ah from Shimon in Shevi'is, Reuven may eat from the trees that hang outside the field. He may not enter the field, even though it is Hefker. This is a decree, lest he tarry on the field after eating the produce. The Torah made the land Hefker only as long as there is produce there.

Question (Kesef Mishneh): Why does the Rambam rule stringently like R. Shimon? This is a mid'Rabanan matter. He should be lenient! It seems that the Mishnah connotes like R. Shimon, for it says (Stam) he may not enter the field. This connotes that he may not enter the field at all.

Radvaz: The Rambam rules like R. Shimon because his reason is Kolel. (It would seem that this means that his reason also explains another Mishnah (of a Choleh, 38b). However, the Rambam does not rule like R. Shimon regarding a Choleh. Perhaps it means like the Kesef Mishneh explains, that his reason totally forbids, like the connotation of the Mishnah.)

Rambam (Hilchos Shemitah 4:24): It is a positive Mitzvah to abandon everything that the land produces in Shemitah - "veha'Shevi'is Tishmetenah u'Ntashtah." One who locks his vineyard or fences his field in Shemitah was Mevatel this Mitzvah. The same applies if he gathered the produce into his house. Rather, he must make everything Hefker, and all have equal rights to it - "v'Ochlu Evyonei Amecha".

Kesef Mishneh: The Mechilta says 'one should not say that since the purpose of Shemitah is in order that the poor will eat, I will take the produce into my house and give it to the poor. Rather, one should make openings in his fence (to allow people to enter). However, Chachamim made a fence, for Tikun ha'Olam (to fix matters). I.e., one need not breach his fence. Surely one may not fence off his field. One who does so transgresses and is Mevatel a Mitzvas Aseh, even if he intends to make it Hefker later.

Ri Korkus: It is forbidden to buy from one who fences his field and guards the produce. If one did so, Tosfos (Yevamos 122a) forbids the produce; Tosfos (39b) challenges this opinion. Rashi (in both places) permits, and the Ramban proved that Rashi is correct. It seems that the Rambam agrees.

Perush ha'Mishnayos: He may not enter because the land is not Hefker in Shemitah. There is a Heter only to walk in it to eat produce. If he tarries there while not engaged in eating, he benefits from the other's property.

Perush ha'Rosh (42b DH b'Omdim): (According to Ula,) perhaps no one may enter the field to pick fruit hanging over the border, since one can pick without entering the field. Or, perhaps it is permitted to others, and forbidden only to a Mudar Hana'ah.

Poskim and Acharonim

Kehilas Yakov (Nedarim 28): How does his vow forbid what will grow in Shemitah? It never belonged to him! Also, one cannot forbid Davar she'Lo Ba l'Olam on someone else! We must say that he forbids the land and the trees now, and automatically Giduleihem (what grows from them) is forbidden. Why is the Rambam (Nedarim 5:16) unsure whether or not Gidulim are forbidden to a Mudar Hana'ah? Perhaps he distinguishes Gidulim of the ground or trees, in which the matter that was forbidden is intact, from Gidulim of seeds that disintegrated.

Question: We must say that in Shemitah, also the trees themselves are Hefker. If not, even in Shemitah one could forbid what grows from them. However, a case occurred in which someone planted his vineyard in Shemitah. R. Akiva ruled that the Torah made it Hefker, so he cannot forbid it (Kil'ayim 7:4,5). R. Shimshon explains that the grapes remain permitted, but the vines are forbidden if they grew one part in 200 (of their former size). They are not Hefker. Also the land is not Hefker; the owner may build on it or dig in it. It is Hefker only for taking the produce!

Answer (Kehilas Yakov DH veha'Nireh): The land and trees themselves belong to the owner, but their ability to make things grow is Hefker.

Chazon Ish (Shevi'is 19:21): Makos 21b establishes the Mishnah to be like R. Akiva, who obligates for Mekayem (keeping around) Kil'ayim. Acharonim ask that R. Akiva would exempt in Shevi'is, for he expounds "Karmecha" (and the vineyard is Hefker in Shemitah)! We can answer that the vines themselves are not Hefker.

Avnei Milu'im (72:2 DH Ela d'Ika): The Gemara asked why the Mudar Hana'ah cannot enter the land. What was the question? The Mudar may enter to take produce, but he is not better than a renter. A landlord can forbid property on the renter, because Konamos uproot Shibud (a lien)!

Answer (Hagahah (9) in Avnei Milu'im): The Torah made the land Hefker for taking produce. This is stronger than a renter's lien, in which the land itself still belongs to the owner.

Kehilas Yakov (ibid., DH v'Emnam): This means that in Shemitah, others acquire the land itself for the sake of entering to take produce. It is not a mere lien.

Question: Konamos uproot even this! A husband owns the Guf of Nichsei Melug for Peros. Nevertheless, if his wife knocks out a limb of her slave, he goes free, for freedom and Hekdesh (or Konamos) uproot liens (Bava Kama 90a)!

Answer (Kehilas Yakov): Freedom and Hekdesh do not uproot a Kinyan Peros. There, freedom takes effect on her Kinyan ha'Guf, and there cannot be a Kinyan Peros on a free man.