Since Google Voice search (Google Now) is simply accessible by saying "OK Google" on Galaxy S5, you may find the Samsung's intelligent assistant, S Voice, no that useful (if not useless at all). To disable S Voice on Galaxy S5 and avoid running the service by double pressing the Home Button, do as follows:
1. Go to App Drawer and open S Voice application
2. Tap the 3dot icon on the top right, and then select "Settings"
3. Under Wake-up settings, make sure to unmark the "Open via the home key" option
If you want to completely get rid of S Voice on Galaxy S5, you also have the option to Disable the application, by pressing and holding the app's icon and then drag & drop it to the Disable area on the top.