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 ****on 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.