No, I don't think they have the right to prevent the kkk from meeting in a park so long as they were not violating any other laws or ordinances. That is speech, and I believe in free speech and the first amendment.
Where my opinion is that license plates don't fall under free speech because no one has the right to have public funds devoted to creating you a license plate. You do, however, have the right to buy your own flag or bumper sticker or even paint your entire truck like a confederate flag if you want.