I don’t typically use Bluetooth, so I went looking for a setting to disable it on Ubuntu startup. After finding nothing, I turned to Google. Google found many ways, none of which seemed very friendly (things like sudoedit /etc/... to add a line to kill the service after it started, or add the kernel module to a blacklist, etc.). Then I came across a way to stop the Ubuntu Bluetooth service from being started by adding an override to the init script:
sudo sh -c "echo 'manual' > /etc/init/bluetooth.override"
This is so much cleaner than the other ways; the service never starts, no config files are edited, and the module is still loaded for the rare case I do need Bluetooth.