Esta resposta a essa pergunta foi encontrada aqui Eu colo abaixo do relevante.
Você precisará adicionar uma entrada ao registro do Windows para desativá-lo.
simply add the following dword entry in the registry as this link describes http://dev.chromium.org/administrators/policy-list-3#IncognitoModeAvailability
Windows registry location: Software\Policies\Google\Chrome\IncognitoModeAvailability
Supported on: Google Chrome (Linux, Mac, Windows) since version 14 Google Chrome OS (Google Chrome OS) since version 0.14
Description: Specifies whether the user may open pages in Incognito mode in Google Chrome. If 'Enabled' is selected, pages may be opened in Incognito mode. If 'Disabled' is selected, pages may not be opened in Incognito mode. If 'Forced' is selected, pages may be opened ONLY in Incognito mode. 0 = Incognito mode available. 1 = Incognito mode disabled. 2 = Incognito mode forced.
OR have this reg file do it for you http://www.mediafire.com/file/dylcbs7aqpjc9vl/incognito diable.reg
Eu não tentei, porque não tenho o Windows 8. Embora pareça que está funcionando.