by Davo Smith.
If the process is:
- student submits
- teacher gives feedback
- student submits new files
- teacher gives more feedback
- etc.
Then this is exactly what the 'resubmission' feature is for: http://docs.moodle.org/26/en/Assignment_settings#Attempts_reopened
(There isn't a way to prevent students from uploading a file and then immediately deleting it, but with the resubmission feature enabled, they won't be able to delete any of the original files, once a teacher has given feedback).