Cyclists after dusk with no lights

[LordGrover]

To be fair I suppose I should include other vehicles with broken/faulty lights, but the number of cyclists who don't seem to care or realize how much danger they create astounds me.

Is there a subtle, inoffensive way to let them know?
I've seen drivers switch off their own lights in a vain (and probably dangerous) attempt to demonstrate their folly but this doesn't seem like a very advanced thing to do.
Or should I just drive on by and hope tonight is not the night they get themselves and maybe others injured or killed?