How can I get rid of fleas in my crawlspace forever?
The fleas are most likely lying dormant (deep) in the dirt floor, below where the bombs/sprays will get to.

Couple/few options:


Pave it.
Soak the floor 2' down with a chemical like bleach.
Diatomaceous Earth.


Number 1 is most permanent, #2 is dangerous as all get-out (and probably illegal), whereas #3 is cheap and safe.

Diatomaceous earth is (basically) fossilized algae powder.  You spread it on a surface and it will absorb fats out of the waxy outer protection on bugs like fleas, mites, etc. when they land on it or walk across it, dig through it, etc.

Once those fats are gone, the pest dehydrates and dies.

It'll take some time and effort applying and re-applying, but it's cheap at about $40-50 for a 50lbs bag of "Food Grade".

Doing an Internet search for "Diatomaceous earth" and "Fleas" and you'll finds lots of direction on using it for your application.