Same as you, I talk to them.
Ask them questions, like -
in the Noah's ark story, why did God drown a lot of innocent animals? Noah saved no more than seven of each, so God drowned thousands or millions of animal who didn't sin. [Can they sin?]
He is God, so he could have take them all to Heaven for forty days (Noah too, for that matter). Or gifted them all with breath water. Or made them all vanish, and spent another six days making them all again.

I was more polite when I was younger. I would tell them honestly and openly that I intended to gently mock their beliefs and advise them to walk on. I'd then tease them a bit but they usually took the hint.
More recently I have become convinced that religion is the cancer of which the human race will die. I'm less inclined to chat with people who want to give me cancer. (Getting a bit grumpy in my old age!)

If possible, I invite them in and offer something to drink. I always invite them to speak and I listen. I appreciate their passion and we always have a good discussion. I don't hold with either Jehovah's Witnesses or Latter Church of Saints... but I always have questions. I'm not the be all and end all.

nod smile and tell them i'll read whatever theyre offering, except most of the time i dont

I used to open the door completely naked and that seemed to work as they never came back

Answer the door naked
It works they will leave you alone

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 had some JWs at the door a couple of weeks ago. I just said I wasn't interested as I am an atheist. They said OK and walked away. I felt a little sorry for them.

Owh, I thought it was going to be...'how do you deal with knock, knock jokes?' Haven't had a religious door knocker in years; I'll let you know next time it happens, it will depend on my mood

I can't recall any religious people knocking on my door. I've lived in Utah for 2 years and I'm feeling a little rejected. I wonder if my neighbor's put me on a blacklist after a 3 minute conversation as I was moving in???

LDS bugged the crap out of a young lesbian neighbor in Alabama, she was too nice to be assertive even when they talked about homosexuality. I played the asshole card, but that's literally the only time I've had to deal with door knocker religious types.

It's almost like there's an anti-god keeping idiocy away from me???

It depends on what kind of mood I am in. If I am wearing my Mickey Mouse T-shirt, I will patiently listen to them, take a copy of their reading material and thank them for "sharing"
If not...I listen to them, nod in the affirmative, ask questions to make them think I am about to become a believer and then tell them "Nah..I'm good.

If I am feeling naughty, I will listen to them with a confused look on my face and then ask them something uncomfortable, like.."what does your faith teach about masturbation?"

Door knackers for unwanted Door knockers.?

If they have children with them I address the children and ignore the adults. I tell the child the adults are evil and they should get away. Usually I am not bothered again, everyone is happier really.

I answer the door naked or in my boxers. They don't want to see that and I don't want to see them.

In the Netherlands we have stickers (provided by the local gouvernement) that you can put on your mailbox. In my case Yes to charity and No to sellers or religious proselytizing. It really works in the Netherlands and do not get interrupted anymore. It saves you time, frustration and you can evade those silly discussion. So I agree with Chainsmkmtair that a sign may be a very effective way of prevention. Otherwise you can also shock them by answering the door naked if they are too stubborn to respect the sign.

Since I used to be the one doing the knocking, I have some compassion for these folks. It was never a natural thing for me to do and I hated it. I was dragged into it by my parents. Depending on my mood, I either don’t answer the door or I explain that I’m a former and no longer interested.

Depends on how much time is available and my mood.

Some fun things might be asking them if they are Christians. Further inquiring about whether they just talk about Jesus or actually live by an follow hos recorded example can be fun.

If they calim to live by his example, that's when it's time to invite them in with a big welcoming smile. Offer them a seat while you fetch soap, a foot tub with warm water and a towel.

"My feet are pretty grubby. Who wants to be first?" ?

I got a couple of knockers the other day in fact. Although I think they are cuckoo for Cocoa Puffs I treat them just as I would anyone else trying to sell me something I'm not interested in. I politely say no thank you, smile & wish them a nice day.
They are still human beings (albeit brainwashed) but I won't be shitty to anyone simply because I disagree with their lifestyle or choices.

I ask these “fundamentalist door knockers” into my house-lock the door & offer them some tea or coffee then go through their tiny minds like a chainsaw. I torture them with facts and knowledge about the cruel Bible & why their imaginary God is a psychopath. And in our discussions which become an interrogation it becomes very clear why belief in a religion and an imaginary God is a mental illness.

I am very polite and tell them I have my own faith...but i try get rid of them fast. I door knock ( well I did in NY) not atheism...but for political candidates so I understand how awkward and uncomfortable it is.

I used to be polite to the JWs that came by, but they wouldn't give up. My teenaged son told me that he would take care of it next time. When they came by, they asked if he was a christian and he answered, "I am a pantheist." They had never heard of that, so he explained, in great detail, about a panoply of gods, each one for specific things. As they were leaving, he followed them down the driveway, still espousing his 'views' (possibly Nordic, or Greek - not sure), all the way to the road. They did not come back for a couple of years.

Never done this myself but I have a friend who answers the door nearly naked!

Jehovah's Witnesses are the most prevalent in my area. I don't deal with them much because if they approach me, I tell them that they aren't allowed to speak to me because I'm a former JW who was "disfellowshipped". (I'm not, but I tell them this.) It's worked wonderfully because other JW's in my area are then warned not to approach me because they believe I've been shunned by one of their congregations.

I learned this trick from my ex-sister-in-law who actually is a disfellowshipped JW.

Me reply to door knockers is always the same....
I am a practicing Theodolite, thank you anyway.

I must ask why is it necessary to come to your door in the first place? Surely if you had a tiny desire to buy an encyclopedia or seek religion, you would go to the place where a variety of the product is displayed? It can only mean that they are there to convince you of something against your will. Or to buy something you don't need, want, or afford?