I don't remember where I found it (I REALLY wish I did), but I saw somewhere on an official site that making your opponent change their decision because of a bluff is not allowed. E.G. I can tell you're about to use a power and pull a card up out of my hand, obviously trying to make you think it's a power spray, and you decide not to use the power, is illegal. I know it's a bad example, but I read something like that somewhere.
But, like espeonrox said, messing with their own decisions is fine, but you can't change a decision that they've already made. I know this is all unclear and it's a fine line, but I believe it is there.
Maybe you should re-post this in the rulings forum.