Unwanted Purchases/ Purchased by Mistake

My sister recently purchased a movie from my account with my permission. I had a card registered only for the songs I had previously purchased, but i only had a little bit of money left in the card and she purchased a movie with it. I never knew until I tried to download an app and it would not let me and it kept telling me to update my Billing Information. I checked the purchase history and It shows the movie is pending, how do I get rid of the unwanted purchase?