Assessments are automatically closed when all responses have been received.
If you want to close an assessment before all responses have been received, you can either:
Close all assessments
or Close individual assessments
How do I close all assessments?
To close all assessments:
Open the Feedback section
Click Actions > Close assessments
Click Close all assessments
How do I close an individual assessment?
To close an individual assessment:
Click on the name of the person being assessed
Open their Settings tab
Click Close assessment...
Confirm that you wish to close the assessment.