should freedom of speech be absolute?

  There have been numerous times when I've been on the (yes, public) bus, and someone comes on and starts preaching the word of God.  And although I myself am a Christian, I don't really want to hear it and I doubt any of the other passengers on the bus want to hear it either. Even if it is a public bus, should people be forced to listen to some guy tell them that their way of life is not the life that will save them in the end?  Should there be exceptions to freedom of speech?