You can delete one or multiple submissions made to your quiz. This can be useful if you wish to allow a particular person to re-attempt the quiz even when the quiz configuration does not allow re-attempts.
To delete responses:
Go to the Submissions section under the Results tab.
Click the checkbox next to the response you wish to delete. You can select multiple checkboxes, or even all checkboxes at once, by clicking the top most checkbox.
Click on the Take action button.
Click on Delete.