The hostility of not wanting new people around you or any population growth seems to warrant the much more minor reciprocal hostility of forcing that population to island themselves.
I don't think people who insist their city should not grow are hostile. I think they are just ignorant and deluded. They are either not aware of, or choosing to ignore, the fact that their cities are economically dependent on neighboring cities to house their workforces, and to absorb their own natural increases.