Outlook 2011: Re-mapping keyboard shortcuts?
You can't disable it, but you can re-map the shortcut so you're far less likely to hit it. 

Open System Preferences, go to Keyboard > Shortcuts (tab)> App Shortcuts (Left field). Click the "+", select "Microsoft Outlook" from the "Application:" list, change "Menu Title:" to "Send," and change the shortcut to something you aren't likely to hit accidentally, such as Shift + CMD + Option + G," or similar. 



Close and re-open Outlook. Enjoy not accidentally sending unfinished eMail!