Per this helpful post on the Ubuntu forums, I just had to edit the file '/etc/init.d/rc.local':
sudo vim /etc/init.d/rc.local
And add the following line:
rfkill block bluetooth
Although the post suggests putting the line at the end of rc.local, that didn't seem to work for me. So I added it near the beginning and all is well. Now, after booting up and logging in, the Bluetooth panel icon is happily grayed-out to show that the adapter is deactivated.