Is there a way to completely disable mouse clicks while typing (including from external trackpads)? The old option 'Ignore accidental touches while typing' which is apparently enabled by default now does not seem to do anything at all. I'm using MacOS 10.12.2 on a 2016 MacBook Pro, and I have tap-to-touch turned off.
Agreed the setting on by default does not actually disable the trackpad. It just tries to be "smart" about it, ignore accidental touches.
The only way that I have found is enabling "Mouse Keys" in System Preferences > Accessibility > Mouse and Trackpad. Check the "Enable Mouse Keys" checkbox. When it is enabled, you can disable/enable the trackpad by pressing the option key 5 times.
It is not really a full solution, but between that and the "Disable trackpad when external mouse or trackpad is present" option it is as close as it gets.
You can also increase the resistance of the click of trackpad in the Trackpad settings in System Preferences which may help.