If it a thread that you posted and their hasn't been any other posts on it. Go to the bottom left and you will see the edit tab, right click on it. When the screen you should be able to highlight the entire post and then delete. You can do the same with the title and it should be closed!
Now if there are others who posted on it I am not sure if you can do it that way! You can always report it on the bottom left and tell them why you want to close it and the powers to be should be able to do that!!