How about a big sign right out front that reads something like:
Welcome to (insert park name here). May we magically remind you that your world view, while as valid and as real as sunshine (and maybe even tofu) is NOT particularly the ONLY world view out there. In fact, we are willing to bet on it. Keeping that in mind, and hopefully other stuff too, the ONE thing we wish to leave at the door is your rudeness. Not saying you ARE rude, but things do happen, what with the intense heat, high prices, and national snobbery and the like. So please, just a reminder, no rudeness at all. Mmkay?
Disclaimer: If you do find it somewhat impossible (within reason, of course) to leave behind anything you feel you are entitled to, we have security lockers (for a diminutive fee....well, at least compared to everything else) for your (and our) convenience. They are located a mere scooter ride away, and to the right. You can't miss 'em.
EDIT: forgot to mention what I'd ACTUALLY ban....Acronyms....can't stand them.