Once you have completed the import process to bring content from a previous course into your new course, follow these steps to remove any previously posted Discussion Board threads.
1. In your Bb course shell, expand the Course Tools area of your Control Panel and select Discussion Board.
2. Click on the name of the discussion forum that you would like to edit (at this point you may be presented with a pop-up window regarding Annonymous threads, you can dismiss this window and proceed to step 3).
3. Place a check-mark in the Select All box at the top left of the discussion thread list.
4. Click the Delete button.