If you don’t want students to be able to modify their assignment submissions past a certain point, you can lock submissions. Typically you would do that after the due date, or while a multi-stage assignment was in a review and feedback stage.
In Moodle visit the assignment you want to lock submissions for and click View all submissions.
Below the list of students, set the With selected… option box to Lock submissions and click Go.
Click OK in the pop-up to confirm you want to lock the selected submissions.
Any students with locked submissions will now have a note reading Submission changes not allowed. You can always reverse the process the unlock selected submissions.