Every time I go to the grocery store, or get off of work I walk to the bike rack and see a whele with a QR just sitting unlocked.
The first time it happened I wanted to loosen the QR to make a point. Then I thought "oh that idiot will probably just ride it then get in a terrible collision with a car/concrete when the wheel drops out.". So then I wanted to slip a note in their spokes that said "psst, you should lock up your wheel. You know how simple it is to steal one of those?" but I didn't want to tip off any theives. I dunno.
Maybe I should copy and laminate bright flashy cards that say "IDIOT" and just slip them in their spokes.