I have seen Critter Creek on their Web site recommend against liquid sterilents for clay pots, and I think that recommendation would hold for lava rock. I think they recommend the oven at 400F for 3 hours.
Critter Creek Laboratory ("Thoughts on Viruses and...", bottom of page)
I have also heard that running an electric oven thru its 'self clean' cycle is hot enough and long enough to do a good job of sterilizing pots.