I also have this issue and wonder if there is a way to "reject" the assignment? The problem is, I have a student who turned in a weekly assignment about 3 months too early. I've reassigned it, but it still shows as submitted in the gradebook. When I masquerade as the student, it still shows the icon in Grades page like they submitted it and there is nothing prompting them to redo it other than my comments, which hopefully the student reads. The other problem is that, unless I grade it, which I don't want to do yet, it stays on my teacher activity report as an ungraded assignment, which my admin uses for my review.