MacBook wifi keep turning on

Quite the reverse wifi problem here. I barley use the wifi since my internet is usb tethered with my iPhone. But still the wifi keep turning on automatically. Sometimes it happen when I connect the phone, sometimes after a reboot. I guess I could just remove the wifi service in network settings but that seems a bit drastic since I occasionally use wifi. Any ideas what this might come from? the problem seem to have started with Yosemite.

thanks!