Good evening, @kathrynmccullou ...
Unfortunately, removing an assignment submission is not possible in Canvas. There is a Feature Idea here in the Community that you might be interested in checking out, though: Delete submissions by students. If you feel that this Feature Idea aligns with something you would like to see implemented in Canvas, you can rate it and also post a comment to that topic letting Instructure know why this would be important to you in Canvas.
I know this might not be the answer you were looking for, but I hope that it has helped in some way to answer your question. Take care, and be well.
This discussion post is outdated and has been archived. Please use the Community question forums and official documentation for the most current and accurate information.