Select the Discussionstool from the Tool Menu of your site.
Select Delete discussion from the More drop-down menu.
Click the More link next to the discussion you want to delete, then chose the Delete Forum option from the drop-down menu.
You can also click Forum Settings next to the discussion you want to delete.
Then scroll down to bottom of page and click Delete Forum.
Deleting a discussion will remove the discussion and everything associated with it, including all topics and conversations. If you are sure you want to delete the discussion, click Delete Forum to remove it from the site.