Open the Tools menu and choose Empty "Deleted Items" Folder.
Outlook asks whether you're sure you want to empty the folder; click Yes to confirm the deletion. The folder is emptied.
An important part of keeping your messages organized is deleting those you no longer need. When you learned how to delete messages in Part 3, you also learned that messages you delete are in fact moved to the Deleted Items folder but are not deleted from your system. You should empty this folder periodically to reclaim disk space used by these messages.