With wide enough boundaries, they can’t. If a tardigrade, or mad space bacteria, can survive a ship going from the vacuum of space to re-entering earth’s atmosphere at enormous heat and then being barracked by the thick earth asmosphere ,then there’s nothing that can reasonably be done. It’s considered sufficiently unlikely to not be a realistic concern.
To add to what others have said, Even if “something” survives the odds of it being able to live and thrive here on Earth are pretty small. Most living creatures we know of require a lot of additional amino acids, proteins and other various biological compounds to be available to them, and tolerate many others that they don’t need.
An organism from another planet would likely require different ones that don’t exist here, and possibly be poisoned by much of what IS here.
Spacecraft reentry is significantly harder to survive than just “living in space.”
Organisms are usually good at surviving one type of environment very well. Going from the “very cold vacuum of space” to the “very hot and very high pressure entry” is extremely difficult for any organism. Impossible for most.
For things that aren’t exposed to reentry conditions, they are autoclaved (high pressure, high temperature, for lots of time.) Nothing we know can survive that.
In case you haven’t noticed, we ONLY know of life that exists on earth, and if we can kill everything we KNOW of, then that’s generally considered “good enough.”
Furthermore, the organisms that we do know of that can survive conditions like that (for shorter periods of time) aren’t dangerous to humans so we don’t care.
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