To delete an outgoing message, go into the VPOP3 settings, then go to the 'Users' page. At the top of the page will be a line saying Outgoing Message Queue. Click on that line, and this will show you all the messages waiting to go out. You can now select the message(s) in question, and press the Delete button

Note that you cannot delete a message while VPOP3 is currently sending it. In the current versions of VPOP3, if you try to delete a message which is being sent, VPOP3 will delete it automatically if the attempt to send it fails.