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> "not to let planes ... enter airspace"

How?

Airspaces are already a thing in aviation, and pilots need to seek permission to transit between them. This is done by making the request to air controllers via radio.

https://skybrary.aero/articles/classification-airspace

I was interpreting "not let" to mean you would actually stop them entering. You're right, the parent post probably meant "deny permission to enter".

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