Turing off Bluetooth on the Dell XPS 13 is a real pain. Here is a quick guide on turning it off, keeping it off, saving battery power, and keeping your PC a bit more secure.
First disable the adapter, in a terminal:
sudo service bluetooth stop
To check in bluetooth is turned off or on, in a terminal type:
Now we need to keep the bluetooth adapter from turning back on when we reboot. Use your favorite editor (gedit, mousepad, nano, etc) to change the InitiallyPowered entry in your /etc/bluetooth/main.conf file from true to false.
In a terminal:
gksu mousepad /etc/bluetooth/main.conf
Look for the InitiallyPowered entry
InitiallyPowered = true
InitiallyPowered = false
save and exit.