Microsoft Outlook provides option to automatically empty deleted items folder on exit. But if you do not want to enable this feature and still want manually empty deleted items then you can use the Cleanup tools. List below are the steps to manually clear deleted items folder in Outlook 2013 and Outlook 2010
Click on the on the File menu –> Info menu and click on the Cleanup tools dropdown.
Select Empty Deleted Items Folder option from menu list. And Click on the Yes button to confirm whether you want to permanently delete all the items and subfolders in the Deleted Items folder.