Try this -
Disconnect from your internet connection.
Open Outlook
Go to your Drafts or Outbox folder where this message is you are trying to
get rid of.
Click to highlight the message (which cannot be starting to transmit because
you are no longer connected to the internet)
Go to your Outlook Tool bar and delete it, using the 'x' option or go to
file.delete.
I hope this helps you!