How can I stop automatic cassette swapping when the paper runs out?

On your KX driver PCL XL is the printer default language which supports the PCL 6 media type selection. Media type selection will allow the printer to select an alternative paper tray if it is assigned the same media type. By default all cassettes are set to plain paper. Media selection can be switched off by changing the printer control language (PDL) to PCL 5e, which does not support media switching.

Method:

Go to the Printers folder (printers and faxes on XP)
Right click on the printer.
Select printer properties.
Check the printer language (shown in example below)
To change language click on Device settings tab.
Click on PDL button (shown below).

Click on drop down box and select PCL 5e
Click OK and then Apply or OK button to save settings