Apart from blacklisting which has the risk of a lockout in case you
need to reboot and no ethernet is available, you can also unload the kernel module as follows:

sudo modprobe -r -v brcmfmac

But I'm not sure unloading the device drivers is a good idea as I fear the wlan0 device may still draw power and even more so if the device drivers' power management features are not loaded, currently I keep drivers loaded and simply make sure the default route via wlan0 is disabled:

sudo ip route del default via <Gateway IP> dev wlan0

This will make sure all traffic will go via eth0 and (hopefully)
be more stable and so forth for example for the ntp daemon,