Or use the GPS to disable face ID near customs checkpoints and police stations.
The phones wont unlock if your eyes are closed. So that's kinda already a feature. Of course a full "lock out because this is my distress signal" feature would be nifty, but imagine you blink in the middle of unlocking your phone on a regular day and find yourself very locked out. That'd be very annoying.
The blink issue could be avoided by having it wait a second or so to see if your eyes stay closed.