Apple - Fully disable Spotlight Search

First, reboot into recovery by restarting your Mac while holding down Cmd+R. From Utilities, select Terminal and enter the following command to disable System Integrity Protection.

csrutil disable

To check if it is already disabled, run csrutil status.
Now, just run this:

mv /System/Library/CoreServices/Search.bundle /System/Library/CoreServices/Search_disabled.bundle

Then restart your Mac and Spotlight will be completely disabled.
Don't forget to re-enable System Integrity Protection once your are done.