Login as root.
$ xinput list
You will see results similar to
⎡ Virtual core pointer id=2 [master pointer (3)]
⎜ ↳ Virtual core XTEST pointer id=4 [slave pointer (2)]
⎜ ↳ SynPS/2 Synaptics TouchPad id=15 [slave pointer (2)]
⎜ ↳ Logitech USB Receiver id=11 [slave pointer (2)]
⎜ ↳ Logitech USB Receiver id=12 [slave pointer (2)]
⎣ Virtual core keyboard id=3 [master keyboard (2)]
↳ Virtual core XTEST keyboard id=5 [slave keyboard (3)]
↳ Power Button id=6 [slave keyboard (3)]
↳ Video Bus id=7 [slave keyboard (3)]
↳ Video Bus id=8 [slave keyboard (3)]
↳ Sleep Button id=9 [slave keyboard (3)]
↳ USB 2.0 UVC VGA WebCam id=10 [slave keyboard (3)]
↳ Asus WMI hotkeys id=13 [slave keyboard (3)]
↳ AT Translated Set 2 keyboard id=14 [slave keyboard (3)]
The touchpad id is 15. To disable it:
$ xinput set-prop 15 "Device Enabled" 0
to re-enable it
$ xinput set-prop 15 "Device Enabled" 1