In the top right hand corner of your post should be an "edit" link. You can click on that and delete all the text in your post. You might have to leave a period or something so that some text remains, but at least everything you have written will be gone.
I don't know if you can delete the thread itself, unless you are the moderator of the forum where the thread is placed.
“There are two spiritual dangers in not owning a farm. One is the danger of supposing that breakfast comes from the grocery, and the other that heat comes from the furnace.”