If you've added stamps to a student's assignment while grading using Moodle's PDF Annotation Stamps feature, here's how to delete them:
- In the toolbar, look for the arrow button and click on it:

- Click on the stamp you want to remove. A box should appear around the stamp:

- In the upper-right corner of the box, you should see a trash can icon. Click on it to remove the stamp. (second image, below)
- Save your changes.
(Thanks to Ken Task for this solution and accompanying images)