#**See Ludovic Ronsin's answer below about using dtoverlay, a method added in January 2017.**

[This post][1] seems to be talking about this. The answer talks about disabling the drivers by editing the file `/etc/modprobe.d/raspi-blacklist.conf` and adding:

 blacklist brcmfmac
 blacklist brcmutil

---

Alternatively, you could use `crontab -e` and add:

 @reboot sudo ifdown wlan0

So that command runs at each boot. 

Hope this helps, I'm still waiting for my Pi 3 to arrive.

 [1]: https://www.raspberrypi.org/forums/viewtopic.php?f=63&t=138610