You can exclude the reviewer's scores by scrolling to the People section, clicking Manage --> Reviewers, and then un-mapping that reviewer from your evaluation groups like in this screenshot below:
This will hide that reviewer's scores until you re-add them to the evaluation groups. Please do NOT remove the reviewer from the project completely, as this will delete all of their scores in the project.
Please note that once you un-map a reviewer from evaluation groups, they will not be able to continue their scoring until you re-add them. The reviewer will not lose any of their progress. Your reviewer will see this if they try to access their scorecard when un-mapped: