Very simple, just install:
sudo apt-get install gpointing-device-settings
Select TouchPad off.
For some reason this doesn't do it or you experience that it re-enables it self, try this:
xinput set-int-prop "SynPS/2 Synaptics TouchPad" "Device Enabled" 8 0
To find the name of your device run:
hl@hl-laptop:~$ xinput list |grep -i touchpad "SynPS/2 Synaptics TouchPad" id=7 [XExtensionPointer] Type is TOUCHPAD