Responsa for Bava Batra 42:13

Q. A was pursuing his craft in town T when B began to pursue the same craft. A demanded that a ban be issued [in accordance with the talmudic law of competition] against any one besides himself who will engage in that craft. B, however, restrained him from such action until a decision, as to the legality of such a ban, be issued by a Rabbi. Subsequently A bribed the ruler of the town who issued a decree forbidding anyone, but A, to pursue that particular trade. A again demanded the issuance of a ban, claiming that the Jews are bound to abide by the law of the secular government. B, however, objected to the issuance of such a ban until a decision was rendered by a Rabbi.
A. If B is a resident of A's town,^*The author uses the term Bar-meboo, a resident of A’s alley, but he is only using the talmudic expression. The decision, however, is that in his own house B may pursue any craft he wants; while A may restrain B from settling next to A and pursuing A’s craft. A can not restrain him from pursuing his craft. The secular government, according to Jewish law, has no jursidiction over trade competition; therefore, the Jews are not bound to abide by their decree in such matters.
SOURCES: Pr. 677.

Q. A was pursuing his craft in town T when B began to pursue the same craft. A demanded that a ban be issued [in accordance with the talmudic law of competition] against any one besides himself who will engage in that craft. B, however, restrained him from such action until a decision, as to the legality of such a ban, be issued by a Rabbi. Subsequently A bribed the ruler of the town who issued a decree forbidding anyone, but A, to pursue that particular trade. A again demanded the issuance of a ban, claiming that the Jews are bound to abide by the law of the secular government. B, however, objected to the issuance of such a ban until a decision was rendered by a Rabbi.
A. If B is a resident of A's town,^*The author uses the term Bar-meboo, a resident of A’s alley, but he is only using the talmudic expression. The decision, however, is that in his own house B may pursue any craft he wants; while A may restrain B from settling next to A and pursuing A’s craft. A can not restrain him from pursuing his craft. The secular government, according to Jewish law, has no jursidiction over trade competition; therefore, the Jews are not bound to abide by their decree in such matters.
SOURCES: Pr. 677.

Our masters, the community of Nuremberg, take notice that my relative, R. Abraham, came to me as a representative of his community. The latter complains against members of your community who came to their town (the town of R. Abraham) and are doing business there. You have the power to stop this evil practice, but you do nothing about it. You know that such practice is prohibited by the Torah as evidenced by the talmudic decision (B. B. 21b) that an inhabitant of one city may restrain inhabitants of other cities from competing with him within his city limits, and other similar decisions (ibid. 22a, 21b). Furthermore, were we unable to prove that the Talmud forbids the practice mentioned above, we should have adduced such prohibition by simple logic, as we positively cannot imagine that it be permitted; for otherwise life would be unbearable. The rich would do business outside their own community, in all the settlements, and would thus increase the volume of business there without helping the Jews of those settlements to carry the burden of increased taxation directly resulting from such increase in business. You, the community of Nuremberg, must correct this injustice and must save the distressed from their oppressors. No one but you can comfort them, for no one else can mete out justice within your sphere of influence. Therefore, you should use all the power of coercion you command to prevent your members from going into other towns and competing with the Jewish inhabitants there; by so doing you would serve as a good example to other communities.
SOURCES: Pr. 983; Mordecai Hagadol, p. 306a.