In most cases you can delete the stuck message if you set Outlook offline by going to the Send/Receive tab and clicking Work Offline.
Wait about a minute or so for the spooler to shut down or close Outlook and reopen. Go to the Outbox and delete the message then put Outlook back online using the same steps of going to the Send/Receive tab and clicking the button again to deselect the option and bring you back online.
If you are unable to delete the message while in offline mode or cannot go into offline mode, you'll need to change your default delivery location.
- Add a new PST using the Home, New Items, More Items, Outlook Data File menu.
- Set this file as the default data file in your Account Settings.
- File, Account Settings, Account Settings, tab Data Files
After restarting Outlook, your original pst-file should show up as the secondary mailbox folder. Browse to the old message store, delete the message from the Outbox and repeat steps 2 - 5 to restore the original PST as the default delivery location and close the new PST. Make sure when deleting the message, to also delete it in the Deleted Items folder.