Can you not disconnect your machine from the internet, open mail then remove the emails from your outbox ?
I'm not a mail user (assuming mac mail) neither do I use FM's native emailing ability but I would of thought it would put any emails in the outbox ready to send, therefore without an internet connection it would simply do nothing ?? allowing you to do what needs to be done.
You could also try going to your Library/Mail/Mailboxes folder and trash your outbox.