turn off auto-connect from the default Audio Master bus: View/Buses..., select Audio/Master: uncheck Auto-connect, Update. Close.
save session to a dummy name and/or filename (you can erase the file any time later).
voilá :) from that moment on, the (don't-)auto-connect behavior for every new sessions should be remembered.
for existing saved sessions, be that under NSM or whatever, you'll have to turn auto-connect from all audio buses explicitly one by one (in View/Buses...).
yes, there's a one-time workaround though:
for existing saved sessions, be that under NSM or whatever, you'll have to turn auto-connect from all audio buses explicitly one by one (in View/Buses...).
hth.
cheers