NetGalley members have the option to Withdraw a request if it is still pending.
When a member withdraws their request, you will see the number of pending requests reduce by 1, and their request will no longer appear in your Manage Requests list.
On the Title Feedback page for an individual book, you will see a count of Withdrawn requests under "Activity".
If a request is withdrawn, members will not be able to re-request the book, and will only see the Wish for it! option going forward. The only way to override this is to send the member a widget invite, which you can read more about here.