Step 1: Go to the grade book.

Step 2: Under the View tab and Grader Report select the box next to the student whose Quiz attempt you want to remove

Step 3: This takes you to the User Report.

Step 4: Select the Quiz in question

Step 5: The next screen tells you how many attempts the student made and shows you each attempt.

Step 6: Select the attempt/attempts you want removed and the hit the Delete selected attempts button.

You can do the same thing by going to the View tab and User Report. Once inside the User Report, select the drop down box and choose which student you want to make changes to.