Deleting incorrectly submitted assignments
Open the submitted assignments grading page and hover over the file that has been submitted
At the bottom left hand corner of your browser it should give you the full url to that file including after the word assignment the assignment id (i.e. 201) and the student id (i.e. 56), e.g. /assignment/201/56/submittedfile.doc
Open the files directory in a new tab or window and navigate into moddata/assignment/201/56/
Right click on the submitted file and save to desktop (just incase something goes wrong)
Now delete the folder and the file in it (/56/)
There is still a row in the database that says the student has submitted a file so open the database and the assignment_submissions table, search for the assignment id (201) and userid (56)
Delete the row
Go back and log in as the student to check they can now upload a file.