The easiest of course is to just boil it.That not only kills everything that may be on it,it also releases the tannin's that tend to turn your tank into tea-water.
I do remember a few years back of reading where someone baked theirs in an oven at 200 degrees for most of the day-the inside of the wood has to hit the same heat temps as the outside.You just have to keep watch that nothing sparks up.

I've actually done that with Tree Branches for my birds before and it seemed to have worked fine.
Do you have any idea on what type of wood it is?