"Nuke them from orbit, it's the only way to be sure..." - Cpl. Hicks

Just kidding, boil the ever living heck out of them. Everything you never wanted to know about wood for amphibian vivaria can be found here:
http://www.caudata.org/cc/articles/wood.shtml

While written with newts and salamanders in mind, the rules are the same for frogs, toads, and any other amphibian.