Same place in all 072 and 073 versions.
Machine status > Tools > Device settings > Paper Management > Tray Settings
choose the one you don't want to ask anymore and set it to Dedicated
If they ever want to change the tray settings they would go to exactly the same place and hit Edit to change it
How can I turn this off? I remember being able to do this, but now that the firmware is to 073 version, I cannot find the setting anywhere.