If the dead rock was your rock and you know what was on it before:
Place it in a bucket with saltwater and power head for a week or two to clear out any dead critters, and then add to the tank.
If it is dead rock from someone else:
Boil it first, cool it off with a good rinse and add it to the tank. I don't like bleaching, chlorine gets into the tiny nooks and it will be hard to rinse it clear.