Before the Anniversary Update, there was an on/off switch that you can use to turn Cortana feature on and off in Windows. Not anymore after the upgrade. However, the setting is still available and here is how you can dig into it.
If you are on a computer running Windows 10 Pro or Enterprise edition, open Group Policy Editor, navigate to the following place and disable the Allow Cortana setting.
Computer Configuration → Administrative Templates → Windows Components → Search
If you are running a Windows 10 Home edition that doesn’t have Group Policy Editor, you can still disable Cortana with a manual registry tweak. Open Registry Editor and find the key at the following location, and then create a new DWORD32 value named AllowCortana and set it to 0.
Note that if you don’t see the Windows Search key, you will need to create it manually.
In either case, you will need to log off and log back in to see the change taking place. The change applies to all user accounts on the same computer. Now, after logging back in, you will notice the text in the search box has changed from “Ask me anything” to “Search Windows”.
Clicking on the search box brings up only a simple prompt, “Start typing to search for apps, files, and settings”.