You can either make the boot loader give the
b43.blacklist=yes option to the kernel automatically, or you can add the
b43 module to the blacklist file.
b43.blacklist=yes Every Time
All boot options are set the same way, so you can follow these instructions but with
b43.blacklist=yes instead of
nomodeset. Or you may prefer this excellent guide. (Or this.)
To summarize, here's an essentially identical way to what's given in those sources:
Hold down Shift while booting. When the GRUB menu comes up, press e to temporarily edit the first entry. Add
b43.blacklist=yes to the end of the line that begins with
linux, then press Ctrl+X to boot.
Once booted and logged in, open a Terminal window (Ctrl+Alt+T) and run these commands to back up and edit your boot loader configuration:
sudo cp /etc/default/grub /etc/default/grub.old
gksu gedit /etc/default/grub
In the text editor that opens up, find the line that says:
Change it so it says:
GRUB_CMDLINE_LINUX_DEFAULT="quiet splash b43.blacklist=yes"
Save the file and quit the text editor.
Back in the Terminal, run
sudo update-grub. Once that's done, reboot to verify it's working.
If you ever need to undo the change, you can run
gedit /etc/default/grub and remove
b43.blacklist=yes from that line, or you can restore the backup (overwriting the new file) with
sudo cp /etc/default/grub.old /etc/default/grub. Either way, you must run
sudo update-grub again to apply the changes.
b43 Module to the Blacklist File
Custom boot options are not the only way to prevent kernel modules form loading. Another way is to add them to the blacklist file. Usually, this file does not exist initially, and you must create it.
A complete, general procedure is here. But you can follow this shorter procedure:
Boot using Step 1 above (under Booting
b43.blacklist=yes Every Time).
Open a Terminal window (Ctrl+Alt+T) and run:
echo blacklist b43 | sudo tee -a /etc/modprobe.d/blacklist-b43.conf
Reboot to verify it works.
If at some point you want to undo this, you can run
sudo rm /etc/modprobe.d/blacklist-b43.conf. (Then to load the
b43 module, you'd reboot, or run
sudo modprobe b43. You're unlikely to want to do that though, since as you said you don't have the
b43 module at all.)