You can require students to manually finalise their submission. There are two options for this; accept the submission statement, which is not customisable, and/or click a Submit button.
If the submission statement is required, it is not possible to clear the submission page until it has been selected.
If the Require students click submit button is selected, files will be uploaded, but they will remain as draft and they will not appear as submitted in the gradebook and it is not possible to mark them.
YOU SHOULD ALWAYS INFORM STUDENTS THAT EITHER OF THESE TWO OPTIONS HAVE BEEN SELECTED IN THE DESCRIPTION WHEN YOU SET UP THE ASSIGNMENT! Because someone will always miss this information.
This is what a submitted and unsubmitted assignments look like in the gradebook:
if an assignment is not submitted, then select Edit > Prevent submission changes. This will submit the assignment.