Many years ago, like in another lifetime it seems, I was one of those doorknockers. Really you don't have to try to shock anybody or be rude. They are entitled to their beliefs too. If you just say you are committed to your own religious beliefs and not interested they will leave.

Two fun stories:
I was working in the garden with my sister and nephew when 3 JWs started coming down the back path. This was about the time that Tim Burton's "Mars Attacks" had come out. The language of the Martians was a loud and grating "ACK! ACK ACK!", which we found hilarious, and would often use in response to one another. We looked at one another in silent accord, and began a spirited ACK conversation. I then turned to our uninvited guests, and with arms outstretched, greeted them with a ferocious volley of "ACK! ACK ACK ACK!!".
Those witnesses can move pretty fast when motivated.
The second time, also with Witnesses, I was more direct. I was cutting up downed tree limbs with a chainsaw. When they got about halfway down our long driveway, I started the saw, and just stood there, expressionless. They retreated slowly, making no sudden moves.
Living rather deep in the jungle as I do now, I thought the opportunities for such fun were over. But just the other day, I found an invitation to the local JW church for their Easter service, in a plastic bag (#rainforest) affixed to my gate post with a rubber band. I'm SO sorry I missed them, but it does give me hope! ?

I have signs posted that read no solicitation no trespassing all violators will be prosecuted to the fullest extent of the law

Posted this elsewhere but friends of a friend were butchering some chickens when Mormons showed up at the door. The guy, still wearing a bloody apron and carrying his bloody knife opened the door and said "yay, Christians!" They ran for a block.

With this:

Here's a great one.
I used to work for a small town newspaper up North and every week we would call this guy, Mr. Green, to get the weather report. Now, it was a bit of a joke to see who would call him, because he was a HUGE weather fan. He could tell you, off the top of his head, the temperatures etc. for this day in the year going back decades! We would be on the phone with him FOREVER!
Anyhow, some JWs came to his door one time and their opening gambit was to mention the weather. Well, 45 minutes later the JWs were looking at their watches. "We really have to be going ..."

I usually just tell them I'm Catholic. I did engage with a couple of Mormon missionaries once. They really wanted to chat with an atheist, having never met one before. Nice kids.

I tell them I am a Buddhist and I offer them dried fish.

I invite them in and debunk the Bible for them.
It always gets me on their "no knock" list

I honestly quite like Mormons. I think they're generally quite invested in their faith and community, and often do good work. There are strong ideological differences between us, but I'll invite Mormons in for a cup of water if they've been walking around all day. I like to explain that I am atheist, and that I have no interest in their religion, but that I am happy to share some stories with them or a little bit of food.
Generally, I hope to show them the kindness they don't often see, even in self-identified Christians, and it brings me pleasure to show them a little kindness.

I believe in being civil, like you, and I sometimes have a conversation and explain my views but it depends how busy I am. I make it clear that I am a practicing atheist and i think it is good for them to hear that there are such totally opposite points of view.

I tell them I'm busy worshipping Satan. That usually tackles that problem.

I was taught to never interrupt, but politely waiting for them to finish talking doesn't work, and trying to debate with them is a waste of my breath and time, so I just talk over them, say "not interested", and then shut the door.

Turtle myself inside my shell until they go away.

I do as my granny did let them in make tea and biscuits and agree with everything and usually end up (because in a previous life i was a lecturer in person centred counselling) being a comfort by actively listening to their tales of woe about the church - Many of the people whom i have seen have been vulnerable adults who have me horrific stories of abuse in their church, which htey don't seem to understand thenselves & I just hope I have been humane enough when they go on their way - We have had in teh past quite a litany of sexual abuse from priests in this area and its all under wraps and no one will really tell. But i have lived in sheltered accommodation for the last eight years so its not a place people easily come to here.

Totally caught off guard yesterday morning at like 10am when 2 women showed up at my door. I literally live in the middle of nowhere. I was polite but my son was standing beside me. Just leave me alone and go push your agenda on someone else.

Im always ready with my principia discordia, a cold pint and my tattiest undies. Given that I already look like the kind of guy you wouldnt trust with your kids, conversations have been brief and usually over the intercom

I conpletely forgot that I could also offer them a beer and an anti-depressant. Who knows? It could turn into an orgy of some kind.

I love the witnesses etc. coming to my door. Their faces usually drop now as they often remember me. You would think they would strike my address off their list.

I am always civil but usually get straight to it as they usually start to get away pretty fast.

They usually start with a ‘ do you think the world is getting better or worse’ kind of question. I explain how the world is demonstrably getting better. Less wars, far less death in those wars, less poverty, healthier people living longer etc.

After that I usually go with the ‘I am more moral than your god’ and point to the god ordered rape of King David's wives in 2 Samuel 12:11:-

 “This is what the Lord says: I am making trouble come against you from inside your own family. Before your very eyes I will take your wives away and give them to your friend, and he will have sex with your wives in broad daylight.

I then ask them if they believe in rape or slavery. They usually say they do not. I ask them why they believe that as it is not in the bible. In fact, quite the opposite. I point out that their morality is much more similar than my morality and it has nothing to do with their bible. It has to do with secular morality evolving over time.

They are usually backing away from me like I am a crazy person at this time saying something like ‘’well, we’ll be off as we obviously can’t convince you’’. I say, ‘’ but you haven’t even tried. If you’re correct and I go to Hell, on your heads be it’’.

They are usually running by this time

The most fun I've ever had was with the Mormons the last time they stopped by. I enthusiastically thanked them for the Book of Mormon. They asked me if I was familiar with it. I gushed about how I loved it and it was my absolute favorite Broadway show. If you haven't seen the show, you really shoud.

You were certainly more polite and accommodating than I'm willing to be. I haven't had one since I told them I was a witch.

Im not trying to intentionally be mean but usually I start laughing and tell them that they have no idea what they're about to get into..most of the time that's enough.

Politely point to the "Do not knock sign" I calmly explain I don't share their beliefs and wish them a good day. No need to be rude. They're people. Just misguided people.

If I'm not working on my website - I take the time to talk to them. I've only had Witnesses come to my door and when I am able, I invite them to chat on my back deck. I've had some good conversations and even made a friend who I learned a lot from and despite our differences we continue to remain friends to this day 24-years after our first conversation. I don't feel the need to negatively react to door-knockers because they don't know whose door they're knocking on and the fact that they keep doing it despite some frightening encounters demonstrates a great deal of courage.

We never get them which is a shame. I would love to have a polite conversation. I have had plenty of online chats with religious people and they have been very cordial. I do find it throws them a bit when you are nice and polite to them. They seem to be brainwashed that all non believers are evil.