You are Jack and the Boxer's shelf you are looking at is Jill.
You are Jack, and the other Boxer is Jill.
Click under the Box you have put books into
Go to the Box-O-Books area
Click the Pending tab on the Box-O-Books page
In this example, you are Jack, and the other Boxer is Jill.
Clicking Decline Offer now will cancel the offer, and the other Boxer will get an email telling her the previous Box-O-Books offer had been DECLINED