I used to smile and say "I'm not religious", but a friend pointed out that this still confers some degree of social legitimacy on their absurd intrusion into my life.
I decided that a better tactic, when it becomes clear that a doorknocker is peddling religion, is to just silently close the door in their face.
Wondering how many people here agree that this is a reasonable response to unreasonable intrusion, and how you choose to respond to these people.