I actually heard once that if you spray them with something, they lay their eggs just as they are dying & the eggs are immune to whatever you sprayed them with. So eventually they are mutants that just won't die!
We have them ATM too & I'm using the egg stopper spray stuff. It kills the eggs too, so I'm hoping that works.
We are on the coast & have always had problems with them here, but when we lived out west for 4 years, we never had one.
It also depends on your house & the way its built. Ours has garage & storage underneath, then behind the storage room, there's another door that goes under the house, & coz there's all that space, thats where they come from or something. Coz the house is off the ground, but its all closed in under IYKWIM?
Mum & Dad lived in this house & had the same probs, when they moved into a house level on the ground, all was solved. We will be moving in a few weeks & I'm praying the same goes for us!