How do I disable Command+Enter sending e-mail in Outlook 2011 for Mac?

Go to System Preferences -> Keyboard -> Keyboard shortcuts -> Application Shortcuts. Now add Outlook/Entourage to the list and make sure that "Menu title" states "Send". After that choose whatever cryptic shortcut you are sure you will never accidentally hit.

Note that you have to quit Outlook, make the change in System Preferences and relaunch Outlook 2011 for the change to take effect.

If you prefer not to have any keyboard shortcut assigned to the "Send" command (as I do) I suggest using a zero-width space character (set via the below Terminal command). This will effectively disable the keyboard shortcut completely:

defaults write com.microsoft.Outlook NSUserKeyEquivalents -dict-add "Send" "\U200B"

Be sure Outlook is not running when you issue this command, or the preference may not be saved.