It would be nice if that was a feature at least before the reply was posted. I have started replies and then changed my mind - but there doesn't seem to be a way to get rid of it. I either need to post something else ... or just ignore the 'draft' that is hanging there.
So.... if you change your mind before you post, just hit the back button to anything you typed. Then move on to other pages, posts to read etc. Not sure we'll ever be able to come up with a feature to make up your mind to what you want to post before you post it. Ryan's good at the tech stuff but not that good.