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
Ctrl+Alt+T
which will bring up crosh in a browser window. At the crosh prompt type
tpcontrol status
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.
Ramblings of an old computer engineer/developer 
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This did not work for me, my chromebook is a HP 14 and what did work to completely disable was:
tpcontrol set 129 0
To enable:
tpcontrol set 129 5
I’m happy!! 🙂
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tpcontrol no longer works?
tpcontrol is deprecated. Please use inputcontrol instead. What will stop touchpad now on HP Chromebook 14?
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