[askvictor]

You could do this to every passer by and still hardly make a dint; that's assuming you actually have the time and energy every time this happens (which I certainly don't). My compromise is to put my hand out in front of my eyes to both shield my own vision, and also hopefully to send a message to the oncoming cyclist.

A more aggressive approach, when I'm on a bike myself, is to briefly turn on my own bright headlight, pointed onto the oncoming cyclist's face; I don't do this particularly often nowadays, but would be curious to see which is more effective at behaviour change.