In our experience allowing them to chew through the power cabling and then roast in the resulting fire certainly sorts out the offending nest, but may not be a preferred control method.
We have only ever had the one problem which was resolved the same as Peters!
I have heard in the past that expanding foam in ducts before the controller chamber is a way to keep them out, obviously leaving the complication for future cable pulling though.
We have had a few problems with these! In some cases gas grille controller bases have had to be installed to keep them out! If you use expanding foam for duct seals you must also insert wire wool as wadding otherwise they will chew through the foam as an evenings exercise. The duct network is seen as a ready built warren. If you are not worried about the pits (bear in mind weils disease and your duties to employees/contractors) then just seal the ducts leading to the controller. The best way to stop them entering the controller is to make sure that the base seal is kept intact. Especially under transformer plates which aren't usually checked. Also make sure that is does seal the base properly, many are poured in without any regard as to whether it has actually sealed the whole base area. If you still can't keep them out, then you will need to talk to a pest control company who will bait the controller for you (nasty stuff)
We had a similar problem, with our Environmental Health guys baiting the controller, the duct network and with us repairing the chamber lid surrounds where they had burrowed under the contrete collars and eaten through the plastic walls.
They had also eaten through an already quite thick controller base seal, although they never chose to have a nibble on the cables.
On a similar thing though, we have had a problem with ants and have had a couple of nests blow dimming transformers. Again, retrospective application of more concrete, ant powder and some vaccuming out has solved the problems. For now. But they are determined little so-un-so's so it will probably be a never ending problem in reality.
On a related note, we had a problem with rats chewing loop tails. Or at least we thought we had until we found our contractor had fed them across an ACO kerb into the chamber and they were being broken by the highway maintenance contractor rodding the drain. We couldn't understand why the bait was never eaten and no other evidence was found!!