system
Given the following post, respond as if you're commenting on Reddit:
human
[Warhammer 40k] What's to stop someone from smuggling Ork spores onto Earth?

Full disclosure: I only have a limited understanding of the Warhammer 40K universe, but I'm wondering if there are any in-universe defenses against someone bringing Ork spores onto Earth and destroying the Imperium from within. How does the Imperium defend against this threat?
gpt
Ignorance, mostly.  Humans, even criminals, don't want the Imperium to fall and Xeno Scum aren't given free access to Holy Terra.

The few humans who do want to harm the Imperium of Man typically don't understand Ork biology.  They wouldn't know to collect the spores and take them to Earth.

If they did, they would also need to seed them in such a way that removal is impossible.

...and even then, they'd probably just seal the Golden Throne and burn everything else to a cinder.