Spy planes can be shot from the ground, or with another plane. This is closer to an act of hot war :(

Orbital space (around modern Earth) is ex-territorial, so killing a spy satellite would be seen as an act of aggression, not legitimate defense. This holds back "kinetic action" in near-Earth space.

> Orbital space (around modern Earth) is ex-territorial, [...]

That's a historical accident of arrangements on earth (so less useful in the Fermi-paradox / filter debate), and could easily have gone differently. Ie air space could also have been seen as ex-territorial.

Assuming something like air space territories in these other cultures, extending them out to infinity seems... tricky. Wouldn't it imply that the ownership of various celestial bodies changes with time?

My recollection of how it evolved on Earth was the soviet's more asked forgiveness than permission and eisenhower basically shrugged and said "whatever, at least our spy satellites can go over you too"

We can make certain assumptions guided by game theory and ideas about evolution to speculate what aliens might or might not do.

See also how for us earthlings the moon is considered the common property of all mankind or something like that. And that's all well and good as long as no one can actually reach it well enough to make use of it; but I predict as soon as we have useful moonbases, we will move to a conception that's closer to 'possession is nine tenth of the law', if not outright 'might makes right'.

If not de-jure, then definitely at least de-facto.