Completely Disable Touchpad in Ubuntu

To temporarily disable the touchpad while typing just go to
System > Preferences > Touchpad
and check
Disable touchpad while typing
but if you want to completely disable it, you can do the following
In a terminal type:

xinput list | grep -i touchpad

to determine the device ID (in my case, 14). Then disable by typing:

xinput set-prop 14 "Device Enabled" 0

To enable it, type:

xinput set-prop 14 "Device Enabled" 1

From the Ubuntu Documentation: Synaptics Touchpad