Anniversary Update has taken away the Cortana On-Off switch, but its still possible to turn off Cortana completely in Windows 10 version 1607 and version 1709, and you can restrict the start menu search to Windows only, here is how that can be done.
Completely Turning off Cortana in Windows 10 Pro and home versions
I. If you’re using Windows 10 Pro, open Group Policy Editor by typing gpedit.msc in Run dialog
Navigate to Computer Configuration > Administrative Tools > Windows Components > Search
On the right, double click on Allow Cortana and set it to ‘Disabled‘.
Note: Group Policy Editor offers following description for the allow Cortana setting
“This policy setting specifies whether Cortana is allowed on the device.”
“If you enable or don’t configure this setting, Cortana will be allowed on the device. If you disable this setting, Cortana will be turned off. When Cortana is off, users will still be able to use search to find things on the device. ”
II. If you’re using Windows 10 Home, open registry editor and navigate to
HKEY_LOCAL_MACHINE\SOFTWARE\Polcicies\Mirosoft\Windows\Windows Search (you need to create this key if doesn’t exist), create a new DWORD (32-bit) value AllowCortana and set its value to 0.
For you to see ‘Search Windows’ prompt instead of ‘Ask me anything’ when you click on the Cortana Search icon, you need to sign out and sign in back again. [Via zdnet]