Since the Windows ‘Anniversary Update’ to version 1607, there is no easy way to disable the dreaded Cortana.

Instead, a registry update is required.  Add the following key:

HKEY_LOCAL_MACHINE\SOFTWARE\Policies\Microsoft\Windows\Windows Search\AllowCortana

with DWORD value 0. You may need to create the Windows Search key first.

For full details, see the item on winaero.com.