Try https://github.com/jcrocholl/pep8

You could show the output of this to the submitter (somewhat bad -- barrier to submission) or flag them yourself so you can correct them occasionally.

Examples:

  $ pep8 optparse.py
  optparse.py:69:11: E401 multiple imports on one line
  optparse.py:77:1: E302 expected 2 blank lines, found 1
  optparse.py:88:5: E301 expected 1 blank line, found 0


  $ pep8 --show-source --show-pep8 testsuite/E111.py
  testsuite/E111.py:2:3: E111 indentation is not a multiple of four
    print x
    ^
      Use 4 spaces per indentation level.

      For really old code that you don't want to mess up, you can
      continue to use 8-space tabs.