by Davo Smith.
Did you set the assignment so that students have to click 'submit for marking'?
If not, then, unless you manually lock a student's assignment, then they can continue to edit it for as long as they like.
The 'Never' setting means that, once a teacher has marked the work, a new, fresh, submission is not allowed for the student. It does not affect whether or not the student can continue to edit the existing, unlocked, submission.