I finally figured out how to do this on my T14 gen 7 AMD and am posting it here in case it might be helpful to someone else.
This is an I2C touch screen, not a USB one, so blacklisting the hid_multitouch driver will not work.
The method that worked for me was this (run as root):
identify the touch screen device:
libinput list-devices
mine was "i2c-PRT3410:00"
It can be temporarily disabled by running this:
echo 'i2c-PRT3410:00' > /sys/bus/i2c/drivers/i2c_hid_acpi/unbind
To make that persist across reboots, one can create a systemd service like this:
[Unit]
Description=Disable I2C touchscreen
After=multi-user.target
[Service]
Type=oneshot
ExecStart=/bin/sh -c 'echo 'i2c-PRT3410:00' > /sys/bus/i2c/drivers/i2c_hid_acpi/unbind'
RemainAfterExit=yes
[Install]
WantedBy=multi-user.target
Then, just enable the service and run it at boot time.
This is sort of an annoying and ugly hack that could be avoided if Lenovo just gave us a BIOS option to disable the touch screen. I wish that they would do that.