Door to door preachers. Image from Wikimedia Commons.
Do Not Knock stickers are the equivalent of the Do Not Call register, only for people that come to your door unsolicited selling stuff or wanting to talk about their religion. You can get a sticker for free or print your own from the website. Have you got one, or is it something you'd be interested in?
It's just for sales people and religious cold callers. Emergencies, deliveries and people who actually want to talk to you personally would be exceptions. My sticker actually specifies ("no sales people, no religious groups").