Canceling the download does not cancel your payment. The download won't even start until your payment has been accepted. The only way to cancel the payment is to contact Apple and ask for a refund. You may have to wait until you receive the bill to do so, but it won't hurt to call early just to see what they say.
Or, if you do decide you still want the movie you can go to iTunes and check for Available downloads under the Store menu. The download will start again. Heck, it may start the next time you run iTunes. It's actually hard to stop a download permanently once you've paid for it.