If you are annoyed by unwanted visitors knocking on your door (salespeople or people on a mission to proselytise you etc.)
how about a sign like this?

Text from the sign:

"Door Knockers Please Note:
This household charges $50 per minute to listen to sales pitches, religious messages & fund-raising stories.

This charge is payable in advance!

By knocking on this door or ringing the door bell, you signal your agreement with the terms outlined above."