First, give all welcome visitors a special knock to use (like two knocks, three knocks, then two knocks). Then just put up a sign that says:
"Use the special knock you have been given.
If you use an unauthorized knock, I will either not answer or I will answer with a machete in my hand.
Jehovah's Witnesses: if you touch my door I will rape your Lord.
Good day."
Problem solved.
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Congratulations to Rafael Nadal on a Career Golden Slam. - The Mana Sword