As an administrator, you can delete any post that a user has created.
Navigate to the Menu and click on Discussions.
Locate the discussion you would like to delete - you can use the filters on the left-hand side, the search box, or just scroll down the page, depending on how many discussions there are.
Click on the title of the discussion.
Click on the Delete link.
Click on Delete in the popup window.
The discussion will be deleted.