From the View Submissions window you can delete any student submission. You can either delete a single submission or all submissions the student has for the activity.
When you delete a submission, the student can re-take the activity. For example, if the activity is set to allow for only one attempt, you can delete the submission so the student can re-take the activity.
If you delete all submissions, or if you delete the submission that is set to appear in the gradebook, the gradebook displays "---" if the activity is not past due, and displays "0" if the activity is past due. If the activity allows multiple submissions and a student has made multiple submissions, the submission score you specified in activity preferences appears. For example, if you set the gradebook to display the submission with the highest score, and you delete the highest submission, the next highest submission score would appear.
Note: Teaching assistants must have permission to delete student submissions. Use the Preferences > Permissions page to control which functionality is available to teaching assistants.