Yes. There are 2 options for cancelling a PullRequest code review.
A PullRequest code review can be cancelled from the Reviews page in the PullRequest.com dashboard.
Hover over any review with a Pending or In Progress status and a Cancel button will appear. Click Cancel and the review will be unassigned from PullRequest.
Edit the pull request's title in your version control hosting service to include the text [!pr]
Email firstname.lastname@example.org if you have any questions. We're here to help!