But regardless of the why. Let's be more specific: New York is one of the most diverse cities. Let's say you want to make New York less diverse. How would you do that without forcibly relocating people?
I'd start with the why

As in, gimme a few proper non-xenophobic reasons as to why you'd want to make New York less diverse. It naturally went that way, now why would anyone want to reverse that?

Then we can proceed to the hypothetical how's