[quote author=Paul Coleman
What I do: If it says "no doorknockers" leave a flyer. If it says "no junk mail", knock on the door. If it says both, turn around and walk away.
[/quote]
Try this:
I knock, then when they open the door I hold up my hands and say (quickly,before they get a chance to start on me) "I'm sorry, I knocked before I realised you've got a 'no callers' sign, and it seemed rude to just walk away without apologising."
I've never had a bad reaction to that, and people's curiosity is such that they can't help asking "What was it about, anyway?"
Then you're in