Requirement owners, co-owners and requirement contributors can remove reviewers from a requirement if the requirement is not marked as Final. A reviewer can be removed if they have not yet started the review. Removing all reviewers will move the requirement back to Draft status. In the requirement details window, click on the reviewers icon on the right side vertical menu. Click on the next to the users name to remove them as a reviewer.
Users will need to have project permissions to view and edit the requirement to perform this action. Depending on the settings of the user's project permissions, they may be able to view all the project requirements or just a subset of them. Users added to a specific requirement as a contributor or reviewer will be able to view it.