Hi
StopGrowingOlder
Welcome to Tech Support Guy Forums!
Place Outlook 2007 in Work Offline mode:
File > Work Offline
Close Outlook 2007, wait about a minute for Outlook 2007 to fully close.
Restart Outlook 2007.
Immediately access the Outbox folder and attempt to delete the message. If that fails, leave Outlook 2007 in Work Offline mode (there should be a check mark beside Work Offline).
Close Outlook 2007, restart the computer, start Outlook 2007 and attempt to delete the message in the Outbox again.
Let us know if that works for you or not.