Had an opposite problem - turned auto-enable handsfree @ bluetooth connect -- and I wanted it off.
HOWTO:
1. Enable Hands-free mode from notifications menu
2. Hit Settings menu (lower left)
3. Connect to vehicle
4. Select the method of connectivity to auto-enable
You can choose wifi, bluetooth, or none.
I originally thought it would be good to have it auto-enable when it connected to my car's bluetooth. But I don't always 'use' my car's bluetooth. Sometimes I get in my car while my phone is connected to my headset (so I can keep the call going when I stop & get out). Problem is that even though my phone was connected to my bluetooth headset, it 'saw' the car's bluetooth, it enabled hands-free mode, and while I was talking on the call, hands-free mode kept thinking I was asking it to do something. So it was more distracting than anything, sitting over there in the passenger seat, listening in, not understanding, confused, trying to be helpful but talking out of turn, like my 3 year old.