Reading the answers from other users, I see that the "No Cold Callers" sign is the equivalent of what I had once in my door, which basically said:
In this house, we love the Lord, we are catholics.
Unfortunaltely, this sign was removed from our door "it was a sticker, though".
I'm catholic, but not fanatic or devoted.
From my point of view, if you're religious (or have religious/spiritual beliefs), I think that when these situation happen again, you can stand in front of your window or door and reply:
Sorry, I'm not interested and I'm busy right now. Have a nice day (or any similar in those lines).
if s/he keeps talking you, you have no need to pay attention and just close the door and continue with your business.
Sometimes to my home, Jehová's Witnesses knock the door very early at morning, I usually know they're, so, they keep knock the door twice, but as nobody approach to the door, they leave shortly.
When I (for any reason) open the window and in fact there's Jehová's Witnesses, they usually say if I'm interested in the Bible; I cut inmediateley replying that in fact, I have read the Bible.
Replying that helped me to Jehová's Witnesses stop asking me questions and we mutually say: have a nice day.
If you're not religious (or have religious/spiritual beliefs), reply:
I'm not interested and I'm busy right now (plain and simple).