I think if you make the developers life harder and ask them to abandon the pull request and submit another you'll run in to objections. I think the whole idea

I think if you make the developers life harder and ask them to abandon the pull request and submit another you'll run in to objections. I think the whole idea of pushing commits to an open pull request is a good one because you can see the discussion and the commits which come after it. GitHub actually does this nicely by showing it in a timeline where you have a comment and then a commit following it in the discussion thread.