Hi Larry
Whether or not the assignment attachment files are kept depends on the company policy on retaining this information.
It is possible to delete individual files by viewing all submissions and selecting Edit submission and deleting the file. You could select only the users who have been graded. This can only be done on a file by file basis but may be Ok if you don't have many learners.
Another possibility is to include a cutoff date for the Assignment - once this has passed and all Assignments graded then you can select course completion archive . This will store all completion records as historic completions and delete the files submitted with the assignment. You may also want to unenrol the learner from the course so the learner can't do the course again.
If the course is part of a certification the course gets 'archived' when the recertification period starts.
Try this with a test user to confirm this does what you want it to do.
Regards