In the Review tab of the UI. When you add a reviewer and click on “Review Details”, it takes you to a page that says:
Once this review has been read, press “Confirm” to indicate that the review process may proceed. If the reviewer has submitted their review elsewhere, you may upload the file below and then press “Confirm” to proceed.
By pressing “confirm”, the review is then closed and cannot be reopened. There should be a warning that by clicking “confirm”, the editor is closing the review.