@LCannon In that case, wouldn’t I have to blacklist the soundcard? If not, can you give me some additional pointers? All I want to do is to prevent gdm from starting a pulseaudio instance.
I found a solution in https://wiki.archlinux.org/index.php/Bluetooth_headset#Gnome_with_GDM
It consists of:
Prevent Pulseaudio clients from automatically starting a server if one isn’t running by adding the following lines to /var/lib/gdm/.config/pulse/client.conf :
autospawn = no
daemon-binary = /bin/true
Prevent systemd from starting Pulseaudio anyway with socket activation :
sudo -ugdm mkdir -p /var/lib/gdm/.config/systemd/user
sudo -ugdm ln -s /dev/null /var/lib/gdm/.config/systemd/user/pulseaudio.socket
Restart, and check that there is no Pulseaudio process for the gdm user.
I was hoping, however, for a less “invasive” solution.