Debian Linux Handbook

If you don't use wifi connection, you can disable wpa_supplicant.

Disable NetworkManager's use of wpa_supplicant to manage wifi clients:

nmcli radio wifi off

After this, NetworkManager won't try to manage wifi devices as clients. Now you can just disable/stop the wpa_supplicant service without worrying that it will start automatically:

systemctl disable wpa_supplicant

systemctl stop wpa_supplicant

Note that NM might still be managing an active network interface for a wifi connection. To kill this, just do nmcli c to view the interfaces, and then:

nmcli c d wlan0

… for a wifi interface named wlan0.

Source: https://unix.stackexchange.com/questions/306276/make-systemd-stop-starting-unwanted-wpa-supplicant-service//