other than solutions already mentioned by Connie your best bet would most likely be intense
UV light.
In our
microbiology lab we run a sterilization protocol that runs UV for eight hours.
while you would not need such a length of time run, a two bulb or 4 bulb UV light would most likely render all exposed surfaces sterile when run for a minimum of 2 hours.
Like Connie mentioned, most likely the critters are using hide holes in places like the seat, side walls and any other surrounding pieces of the toilet fixture.
If this is the case you would need to create a situation where all these parts are exposed to the UV
lights for the minimum time period (that is if you choose to try this method).