Responsa for Bava Batra 42:10

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.