You're supposed to be able to click them and remove them.
I think there is a bug that I can't figure out preventing it from being allowed sometimes. So far, the three different computers I've played it on have all worked with removing the leaves, but I've had two friends play it and they have both ran into issues with the leaves.
Frustrating.
And thank you. If you're looking for a more Tower Defense type game, I suggest not buying the candy for the trick-or-treaters and seeing what happens :D