Some KILL examples for wpa_supplicant are given in Support for User-Defined Wpa-Supplicant Config thread. Not sure if its killed permanently though?
kill -9
cat /var/run/wpa_supplicant-wlan2.pid&& rm -f /var/run/wpa_supplicant-wlan2.pid
Also they give an example of starting wpa_supplicant, not sure why so convoluted way?
wpad wpa_supplicant -B -P /var/run/wpa_supplicant-wlan2.pid -D nl80211 -i wlan2 -c /etc/config/wpad/sta.conf -C /var/run/wpa_supplicant/