I have a Dell Inspiron 5040, and I cannot disable the touchpad.
The best solution would be if Ubuntu could actually not ignore the keyboard shortcut that disables it.
Nothing did the trick for me, so I made the following little script that works:
touchpad_id=xinput list | grep PS/2 | cut -f 2 | cut -d "=" -f 2
xinput set-prop $touchpad_id "Device Enabled" $1
Well i am not sure if this program could help you but there are a some options to disable the touchpad.
The first option is installing Jupiter, this program is pretty usefull and has other funtions like change the performance modes and obviously disable the touchapad, to install it:
sudo add-apt-repository ppa:webupd8team/jupiter
sudo apt-get update
sudo apt-get install jupiter
Other option is install an applet that disable the touchpad, this is an applet that disable the touchpad:
sudo add-apt-repository ppa:lorenzo-carbonell/atareao
sudo apt-get update
sudo apt-get install touchpad-indicator