To delete an assessment attempt, you will need to first navigate to the course that contains the assessment attempt. From there, click on the Users & Groups tab and click on the Details button beside the user who completed the attempt.
Next, click on Details beside the attempt, then click on the red X. Accept the prompt, and the attempt will be removed. This will free up an attempt if you have set a maximum number of attempts a user has, and it will also remove the attempt from all reports.