Prevent/stop wpa_supplicant from starting

Should be enough to remove the file

/usr/share/dbus-1/system-services/fi.epitest.hostap.WPASupplicant.service

Move it somewhere so you can restore it, rather than immediately delete it. ;-)

Update for 2019

htorque's solution did not work for me.

However, the following did work:

sudo systemctl disable wpa_supplicant