Google Now and S Voice are your default assistants in Galaxy S7 (Edge). To make S Voice stop and use Google Now as your only assistant, you need to turn off S Voice. To disable S Voice on Galaxy S7 (Edge), do as follows:
1. Open the Settings app on your S7
2. Scroll down to the Applications settings and open it
3. Go to the Application Manager settings
4. Under the list of All Apps, scroll down to the S Voice App and open it
5. Simply touch the Disable (Turn Off) button, right under the app's name and logo
6. Confirm the action and you have successfully disabled S Voice on Galaxy S7
Note that, by default S Voice will be launched as you press the home button two times. To turn off S Voice launch via double home button press, do as follows:
1. Open the S Voice (from the App Drawer or just by pressing the home button two times)
2. Touch the 3-dot icon on the top right side of the screen and then go to the Settings.
3. Find the Wake-Up settings and make sure to unmark the Open via the Home Key option