Edit the file /boot/config.txt and comment the line:
#dtparam=audio=on
Now edit the file /etc/modprobe.d/alsa-blacklist.conf and add:
blacklist snd_bcm2835
Finally edit the file /lib/modprobe.d/aliases.conf and comment the line:
#options snd-usb-audio index=-2
Or if you just want the USB Audio to be your primary sound device.