Very simple, just install:

 sudo apt-get install gpointing-device-settings

Execute:

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