disable the touchpad / ultranav on my external keyboard

When I put my laptop in my laptop stand I use an external keyboard - but how do I disable the touchpad on that?

http://www.thinkwiki.org/wiki/Synaptics_TouchPad_driver_for_X gives the final pointer - xinput

xinput list

note the ids of all the devices that look like they could be the touchpad

xinput list-props <ID>

shows all the properties for the device

joerg@mint:~/tmp$ xinput list-props 13
Device 'Synaptics Inc. Composite TouchPad / TrackPoint':
    Device Enabled (97):    1
    Evdev Reopen Attempts (230):    10
    Evdev Axis Inversion (239):    0, 0
    Evdev Axis Calibration (240):    
    Evdev Axes Swap (241):    0
    Evdev Middle Button Emulation (231):    2
    Evdev Middle Button Timeout (232):    50
    Evdev Wheel Emulation (233):    0
    Evdev Wheel Emulation Axes (234):    0, 0, 4, 5
    Evdev Wheel Emulation Inertia (235):    10
    Evdev Wheel Emulation Timeout (236):    200
    Evdev Wheel Emulation Button (237):    4
    Evdev Drag Lock Buttons (238):    0
    Synaptics Off (261):    1

so "Device Enabled" is our friend.

xinput set-int-prop 13 "Device Enabled" 8 0

seems to turn it finally off. Now I put that in a little bash script, and have it run at the beginning of my X-Session.