Ah, I found a much more thorough guide, also using xinput to control the enabled/disabled state:
Sorry to revive this old thread, but I found another solution in Ubuntu 12.04, which seems more robust and is easy to automate.
My problem was, the touchpad would sporadically re-enable upon resume from suspend.
The following commands should be run in a terminal.
First, find the device id that you want to disable:
In my case, the SynPS/2 Synaptics TouchPad has id=18.
XInput devices have several properties, identified by a number (see xinput list-props <device>).
The "Device Enabled" property has the number 132.
Setting property 132 to 0 will disable the device in X:
e.g. in this case I will use the command:
xinput set-prop <device> 132 0
Add this command (with the proper device id) to your autostart applications, and you have permanently disabled the XInput device - it cannot even be switched on using the Fn-F8 key combo.
xinput set-prop 18 132 0