I usually take pictures of them and shame them on Twitter. Nothing like a little public humiliation.

When I used to go to the gym to use the treadmill, I'd always see the motorcycle drivers parking their bikes in the hashmarks next to the handicap spots. I'd take pictures and tweet them TO the gym (a nationwide chain), and also complain to management in the gym.

Also, if my car door hit their bike when I opened the door, not my fault they're parked in a no parking spot. They shouldn't be parking their bikes there.