My /etc/modprobe.d/blacklist-bcm43.conf, looks like this:
If one isn't automagically created for you, you can create one yourself using yjr following command, press Alt-F2 and type:
# Warning: This file is autogenerated by bcmwl. All changes to this file will be lost.
then copy and paste the contents of my file.
gksu gedit /etc/modprobe.d/blacklist-bcm43.conf