You just don't set it up in the first place. When you launch it for the first time you have to tap the button that says "ok, I'm in." Or have you already done that and now decided you don't want to use it?
Press and hold the Home button to open it (or you can go into your app drawer and tap its icon), then scroll down to the bottom on the Google Now screen, tap the three dot settings menu button and at the top of the settings screen turn it off.
Never clicked on "OK". I just get repeated nags.
I wish we could really delete some of the google apps. Seems the only option we have is to uninstall updates. What good does that do?
Seems like phones tied to providers are loaded with junk just like pcs you don't build yourself!