Create /etc/modprobe.d/brcm-blacklist.conf with the following content:
blacklist brcmfmac
blacklist brcmutil
Reboot, and WiFi should be disabled. You may find errors in the system log depending on what method you are using to configure the network.
Another option:
add:
dtoverlay=disable-wifi
/boot/config.txt to disable wifi. I have tested it with a Raspberry Pi 4B. It is also valid for Pi4.
Kevin -- KD9EFV