Post a sign that says, "If this house is a-rocking; don't come a-knocking".

If you answer the door with very little if any clothing on they will learn to leave you alone.

With out clothing on.

Naked.

Stand on the other side of the door and knock back.

I find that a cordial no thank you tends to do the trick. If they persist, simply explained that it is hard to believe in anything without evidence. Remember if they're knocking on your door that typically means that they are true believers. Therefore you have just as much of a chance to educate them as they do you. You have made your decision based off of empirical data and lack of evidence and they have chosen the contrary.

I believe there's an invention that specifically targets this particular situation- it's called "a shotgun" XD

Seriously though, I just politely tell them that I've already got my own beliefs. They can have theirs, and I can have mine. And I don't accept any literature from them unless I think I can get a good laugh out of it XD

I have always wanted to stockpile some atheist literature to give them.

I tell them I once knew an elder from the jehovah's witnesses. i'm all good with that.

I just sit on the other side of the door and knock back in repeat. (I really just made that up but it sounds like a fun thing)

Answer the door wearing nothing but a rainbow wig. I then invite them in, since "the festivities are just about to begin." I honestly don't know what I'd do if anyone accepted and followed me into the house.

Depending on my day - I usually inform them of my lack of interest, take their literature, wish them a nice day. However, if it is disruptive to my day - I can be quite...curt....

Answer it naked

Hi. I might sound mean, but i think life is too short not to have a good laugh.
About 2 years ago it happened to me, i was coming back from a walk with my dogs and saw door-knockers were coming, so i had time to prepare.
I found an old necronomicom look alike book i had when a was a teen, and when they came,i smiled weirdly, book in my hand and i gave them the classic " Hi ! Im a agent of Satan, but my duties are mostly ceremonials. What blood type are you guys ? Wanna come in ? "
About 5 seconds of silence later, they thank me for my time and left. I told them they can come back anytime.Never heard of them again. Kind of miss them

It depends on my mood. Some days I simply politely tell them I'm not interested. Some days I tell them to "fuck off" and then there are those days I'll answer the door butt naked and invite them in. If they are crazy enough to accept my invitation at that point, I'll debate them until I grow bored and then ask them to leave unless on the off chance I perform a successful "conversion" LMAO!

I just tell them I'm atheist and that they should fuck off.

I used to refuse to open the door, but then I figured, it's MY HOUSE & THEY are intruding onto MY space, why should I hide? So now I open the door with a huge smile on my face & invite them in - and without giving them a chance to get a word in edge-wise, I start preaching my Antago-theism to them and ridiculing their beliefs as idiotic rantings of morons, etc .. until they run for the door ..! I've have several grab their crosses & hold them up in my face, as if to fend off my evil, LOL!!

Coming from a Mormon background, I try to send them away with respect. Most door-knockers do not want the respect if it comes with a NO. They will still argue with me about my personal experiences and try to mansplain why things didn't happen that did. So I don't engage, just say no and shut the door.

I simple say that I have way too many more important tasks to be accomplishing than to engage in a battle of wits with a defenseless person.

"Could you help me sacrifice this lamb?"
someone baa'ing in the background

I answer the door in my shorts and a bottle of liquor in my hand.

I am a real Ass, I get in their faces, I yell, and cuss. I try to make them feel as uncomfortable as if I were in their house

I have fun with them. I offer to give them 5 minutes of my time to discuss their god if they give me 5 minutes to discuss my atheism. Then I politely close the door and say 'let's not even waste each other's time'.

My uncle was looking outside his front window a number of years ago. He saw 2 JWs going from door to door, heading his way. It was an older woman and younger girl, maybe 20 or so. They stepped on the porch and he opened the door with "Howdy, gals! Come on in. The old lady's gone and we can have a good time!" They took off as if someone were shooting at them.

I ask a bunch of questions early on that I know they don't like to answer, and they quickly want to leave. I turn it into a game. How soon can I make them feel uncomfortable without just telling them to go away?