Hello Eddie,
Set your Outlook in offline mode. To do so: Click on “Send/Receive” Tab >> “Work Offline”. Now restart Microsoft Outlook and Open message from the Outbox Folder, select the message and Delete it. Don’t forget to enable MS Outlook back On-line again.
To get more information related to this issue, see here: http://www.en.pstemailrecovery.com/blog/email-stuck-in-outbox-outlook.html
Regards