The undo button didn't work for me either, it pretended to but when I refreshed the page my unwanted post reappeared. I then tried the button to the left of the undo button - 3rd in from right end - hover your mouse over it and it says drafts - when you click on it there are 2 options, save draft and delete draft. When I clicked delete draft it worked but I can't guarantee that is the correct way of deleting a post or if it will work again next time, it may have just been chance because I tried clicking all the buttons. LOL, sometimes it sounds like we all have different versions of the site