I am having the exact same problem, even with S-Voice turned completely off in Settings. Did you ever find a solution?
Found a solution which fixed it for me.
Go to Settings, then More, then Location, then My Places, then Car, then Select method - Tap None, OK and Save.
Now the hands-free mode does not turn on automatically.
Found a solution which fixed it for me.
Go to Settings, then More, then Location, then My Places, then Car, then Select method - Tap None, OK and Save.
Now the hands-free mode does not turn on automatically.