You will see a section of text that includes a link for "end the reviews" - click on that link. UPDATE: you now need to search for your book using the search feature and then in the list below choose the "Edit" button.
4
This takes you back to the page where you setup the review. You can now click on the "End Review" button.
5
A warning will pop up telling you that any outstanding/incomplete reviews will be abruptly ended. Assuming you are okay with that you can press "Proceed" to end the review.