I think it would be worth it to at least voice your opinion without violence. I was waiting for my wife in the Wal-Mart parking lot and some doooosh bag parked over the blue handicap lines. Some young thug trying to be a slim-shady impersonator. He and his older woman partner (I assumed it was his WT mother) just walked into the store to go shopping like they do it everytime they go to Wal-Mart. The blue lines are supposed to indicate that you should not park there because some handicap people need the access to their wheelchair lifts. A couple days later a car full of doooosh bags did the same thing at McDonalds. I probably had over 30 different ideas go through my head on how I could "teach them a lesson". All of those "lessons" would be more illegal and more damaging than a bumper sticker. If I had the bumper sticker in hand at the time I would have thought how I could have used it. For instance I could stick it to their face and then punch it several times to make sure it would stick properly. I am glad I have the willpower to not act out my ideas. Please anyone that reads this, do not park like a tool. If you question whether it is a good idea, or too close to your neighbor, find another spot. Do the right thing.
(BTW the people who pulled right next to the handicap tagged vehicle at McDonalds repeatedly door dinged the car next to them trying to get their fat asses out of the back of the car. Imagine if you can the door being pressed against the other car while a 400lb person wrestles their way out of the car causing it to rock like a boat for like 5 minutes. Damage was done to an innocent parties car.)