Follow these steps in order to Lock a reviewer:

1) Select Opportunity > Evaluators > Groups
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2) Select Reviewer Group where the reviews need to be redistributed
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3) Select the Reviewers tab
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4) Mark the Locked? checkbox next to the reviewer that is unable to complete their reviews
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5) Select Update Reviewers
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If you are unable to see the Locked? feature, you may have specific settings in place that are preventing the system from locking.