There was a guy not long ago who would leave his fixie right out in front of my building all day everyday with nothing but his rear wheel locked to his frame with a U lock. I thought many times about just walking off with his bike, but instead I just took his front wheel off (a red Deep V which matched the back) and left it leaning against his frame with a note that just said "learn to lock you bike"

I never saw that bike there again. Hopefully he started bringing it inside with him or at least locking it to a rack.