It would be nice if you could disable the Chromebook trackpad with a simple function key click. But alas its just not that simple. But there is way, click
which will bring up crosh in a browser window. At the crosh prompt type
This will display a list of settings. I'm using a Samsung Cromebook and the setting I need to change is "Device Enabled (111): 1". Next type
tpcontrol set 111 0
then type exit and close the browser widow. You should now find the trackpad not longer functions. To restore functionality just simply repeat the steps above and type tpcontrol set 111 1.