Sorry, Cortana — not everyone likes you. =)
- Launch the group policy editor by pressing Windows + R, typing gpedit.msc into the box, and pressing Enter.
- Navigate to Computer Configuration > Administrative Templates > Windows Components > Search
- Locate the Allow Cortana setting in the right pane and double-click it.
- Choose Disable and click OK
Optionally, without rebooting:
- Press Windows +R and type gpupdate /force into the box and press Enter
- Press Windows +R and type taskkill /f /im cortana.exe into the box and press Enter