Navigate to your Reviewer Group by going to Opportunity > Evaluators > Groups.  Find your reviewer group and click into it.  This lands you on the Details tab, where you can find the Assign Reviews settings.  Select the Reviewers tab to access the "Locked?" setting.

If the "Locked?" checkbox is unavailable on the Reviewers tab of a Reviewer Group, this is due to one of the following:

1) The Reviewer Group is setup for Manual Review Assignments. Review assignments must be manually reassigned from one reviewer to another.
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2) The Reviewer Group is setup to assign reviews automatically, "Every application to every reviewer."  Review assignments cannot be reassigned because each reviewer is already assigned to review every application.
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