Go to system settings, and switch to Airplane mode, then go back to your outbox, select it and will open as if you wanted to add words to it, and just close it. That will put it in your drafts folder, then you can delete it from there. But I also think you can also just delete it directly from your outbox. I just accidently closed it and it went to drafts, then I deleted it... It does work. Goog Luck!