Begin by clicking on the "Grades" tab. In the "Enter Grades" view click on the caret next to the student you'd like to drop a grade for. Click on "Bulk edit exemptions"

Check the box of the assignment/test/quiz/discussion you'd like to drop and click on the blue "Save and Close" button

You will now see a gray "Exempt" box in the gradebook for that grade item.

NOTE: Using this method does not preserve the original score in the gradebook.