To stop using the new Outlook and hide the switch icon in 'classic' Outlook, do the following. N.B. these are under HKCU (Current User Registry Hive) so may have to be run each time a user logs in:
(Firstly, close any Outlook already open). Then:
reg add "HKCU\Software\Microsoft\Office\16.0\Outlook\Preferences" /v UseNewOutlook /t REG_DWORD /d 0 /f
reg add "HKCU\Software\Microsoft\Office\16.0\Outlook\Options\General" /v HideNewOutlookToggle /t REG_DWORD /d 1 /f