It is called Auto Tray Switching and disabling it is a fairly terrible idea that will lead to pretty annoying things.
You will then always need to set what tray you want to use, no matter what, you will have to choose something when printing or copying, and since many apps default to use Auto tray in the program in addition to the driver, you may need to change it in 2 separate places. But if you really do want to shut it off, log in as admin and go to
Machine status > Tools > System Settings > Common Service Settings > Paper Tray Settings > Paper Tray Priority > Change Settings and then select each tray and set it to Exclude from Auto Tray Switching.
Once that is applied, start fixing all the faults that will be caused since everything is set to Auto
In the case of the last driver (The one for a Fiery RIP), you will end up with a weird fault at the machine that says All trays inhibited if you don't manually choose the tray when you send it.