EclectEcon

One of the clearest cases for limiting free speech is the famous phrase from Oliver Wendell Holmes about, "...falsely shouting 'fire' in a crowded theatre... " Another is the instigation to commit murder or other crimes. And a related case might involve urging genocide and supporting those who espouse it. From Melanie Phillips,

This is what happened on the streets of London yesterday. A demonstration organised by Iranian Islamists promoting the destruction of Israel, in which people called for the killing of Jews and waved Hezbollah flags. The police provide protection for such a demonstration on the basis that people are entitled to march provided they are not breaking the law. They ignore the fact that such incitement to murder is indeed against the law; they fail to grasp that a parade of Hezbollah flags is a recruiting device for Islamists; they refuse to acknowledge that such a demonstration is an flagrant act of intimidation by proxies for a terrorist regime which is currently blowing up our troops in Iraq, threatening genocide against Israel and intent on defeating the west in war. Welcome once again to Londonistan.

While this situation falls short of the "clear and imminent danger" criterion for abrogating free speech, I see no reason to permit the demonstrators to use public property while promoting their ideas. After all, freedom of speech does not require that all taxpayers provide scarce resources for the exercise of that free speech.