Turn your monitor upside down and shake the words out of it. Just kidding.....Left click "private messages",click on the topic.Below will be a "delete this message" box.Click on that one and the one below.It should go away.
On the far right side there is a little square box, click on that box for the messages
you want to delete then scroll down to the bottom of the page and where is shows
SELECTED MESSAGES chose DELETE.