Get rid of the substrate and use paper towels at least until the outbreak is going on.
When you treat the tubs treat the paper towels and allow to dry well before placing in the tub.
Lightly spray down the outside of the tub also, and any surface around the tub rack and allow to dry fully before placing the snakes back into the tubs.
You can also spray the floors if they are carpet.
I have treated a few peoples collections and we do preventative sprays occasionally to prevent any possible outbreaks. I have never had to use anything on the snake itself.
It may only take 2-3 treatments if done correctly, or it could take longer if you are missing the places they are laying their eggs.