You Can't Turn Off Cortana In the Windows 10 Anniversary Update (pcworld.com)
Microsoft will release Windows 10 Anniversary Update next week. Earlier this week we listed some of its best features. PCWorld is now reporting about a major change that may annoy some users: once you've installed the update, Cortana can no longer be disabled. From the article: Cortana, the personal digital assistant that replaced Windows 10's search function and taps into Bing's servers to answer your queries with contextual awareness, no longer has an off switch. The impact on you at home: Similar to how Microsoft blocked Google compatibility with Cortana, the company is now cutting off the plain vanilla search option. That actually makes a certain of amount of sense. Unless you turned off all the various cloud-connected bits of Windows 10, there's not a ton of difference between Cortana and the operating system's basic search capabilities.
Yep. You can disable it in the group policy editor if you have pro, or in the registry editor if you don't.
Meanwhile, in MacOS, all you have to do to disable Siri is uncheck a simple GUI Checkbox.