Normally I don't let them power attack an object. Can't take a penalty on an attack roll when one is not made. I normally don't have them roll an attack on something without an AC. No penalty means no gain.
But that's just me. I have seen it argued before on the boards to the point allowing it or not is supported by some interpretations of the rules.
Later
But that's just me. I have seen it argued before on the boards to the point allowing it or not is supported by some interpretations of the rules.
Later