As I do with all of my rocks/branches that I find outside:
1. Put them in the tub with hot water, and add some bleach and let them soak about an hour.
2.Rinse off log/rock until no more bleach smell.
3. preheat oven to 250 degrees, place in oven for about an hour.
You really only have to pick between steps 1-2 or step 3. I do both just to be sure. The oven is probably the safest and best way to do it because it gets the wood to a high internal temperature which kills all the nastys deep down in the wood.
-Brandon