Original discussion can be found here:
https://news.ycombinator.com/item?id=8479958
Hint: you have to uncheck two checkboxes that OS X explicitly tells you about in the very same Spotlight preferences, plus another one in Location preferences.
However, I can understand the philosophy of searching the wen and the desktop in parallel.
Ubuntu does the same (they search Amazon, not sure if also the web in general), and they also got a lot of flak for it.
If one would never even open Spotlight preferences, then yeah, it is not possible to see, enable or disable those preferences. But then one should also not complain that it is impossible to enable or disable these preferences. By that logic, every application that does anything with any privacy implications should have it's primary interface littered with preference toggles to make it completely obvious how it's functionality can be altered.
Having people go to two preferences dialogs just to find out that contents of search box are being sent to USA datacenters is a dangerous dark pattern.