There is a complicated way of accomplishing this with udev rules, systemd service and suspend/resume hooks.

An easier way is to check this:

$ cat /etc/pulse/default.pa | grep switch-on-port-available
load-module module-switch-on-port-available

If the second line doesn’t begin with a # (commented out) then edit the file and insert a # in the beginning.

Now all output will be controlled to what you manually set it to and stay there until you change it.