Organisation Administrators can remove any Reviewer from a request. Also, the Reviewers of a request can remove those Reviewers they have previously added.
This is how you remove a Reviewer
In the Review step, click on the Change Reviewers icon:
Hover over the Reviewer you want to remove and click on the red Cross icon next to their name:
If you don't want to add any Reviewers to the step, click on DONE.