When I want to totally remove something I've started to write, I first back-space over all of the text or, use my "Select All" option with my mouse's right button and then hit "backspace" key on my keyboard. That removes the text but the forum software makes a copy of everything that you have written in a post or thread and has stored it in something called a "Draft". That Draft needs to be removed too.
Like Carbon 6 said, look for the "computer disk" symbol over at the right side of the tool bar at the top of the post. (It looks like an old fashioned "Floppy disc" in an envelope.
When your cursor passes over it, a black box that says "Drafts" lights up.
Click on the symbol and then poke the "Delete Draft" button.
When you do this, and since you removed all of the text in the typing window, everything is now gone so you can move on to some other subject.