to disable the "new look" outlook, and prevent it from ever prompting to be used (i.e., after an update), open terminal and issue the following command. this will remove the "new outlook" toggle, and will prevent it from periodically / randomly switching back to the new outlook (when you have it switched off) after an update. After running this command the toggle will be removed completely, and you won't be prompted to switch anymore.
defaults write com.microsoft.Outlook EnableNewOutlook -int 0