In America, we don't trust our government to know the difference between
acceptable bumper stickers, and unacceptable bumper stickers. So we don't let the government do anything about any writing on any bumper stickers.
The alternative, I guess, would be to let the politicians or some crazy TV preachers or the people who print offensive bumper stickers decide which ones can't be displayed. But if we did that, then the people who couldn't display their bumper stickers would get angry and start shooting at the government people responsible for the unfair censorship of messages.
So, you can do one of a few things:
1) Talk to the person who has the bumper stickers, and ask to remove them.
2) Make your own bumper stickers
---"Promote World Peace, Kill a Pea Brain Fundamentalist"
---"Promote World Peace, Ban Bumper Stickers"
---"Promote World Peace, Get a Muslim Pen Pal" <--- best idea I've had today, actually
---"Promote World Peace, Just Say Yes"
--or make up something you would find more persuasive.
The idiot who drives with those inflammatory messages on his car will be noticed, and most people will think poorly of him. His goal is to vent his bile, and to make other people angry.
If you try to out-do him in his nastiness, then both of you will have lost. As they say, "Don't try to wrestle with a pig -- he actually likes it, and both of you get dirty."
If you demonstrate to your neighbors why you are a wonderful person with deeds instead of words, then they will be better prepared to listen to your words.
So, that's how we do things in America. It's not always pleasant, but it's always ours.
Best wishes, and welcome.