The touchpad seems to be referred to as "mouse" in the xorg.config as well as any regular mouse - the one on my laptop appears in the config-file like this:
Identifier "Alps GlidePoint PS/2"
Option "Protocol" "GlidePointPS/2"
Option "Device" "/dev/psaux"
You could perhaps try and comment out (#) those lines and see if it disables the touchpad. Eventually it'll instead crash the computer when you try to run XServer, so make sure to backup your original xorg.config and know how to revert the changes you made on console...
I hope this helps in any way.