Hi
yBrad
Place Outlook in Work Offline mode:
File > Work Offline
Close Outlook, give Outlook a minute to close.
Start Outlook, access the Outbox and attempt to delete the message.
If you are successful in deleting the outgoing message, place Outlook back online:
File > Work Offline
If the message refuses to delete, leave Outlook in Work Offline mode, close Outlook and restart the computer.
Start Outlook and immediately attempt to delete the message again.
Let us know if that works for you or not.