I generally am pleasant but firm, and tell them not only that I am not interested, but that they are wasting their time knocking on ANY door on my block. After two or three similar experiences in my neighborhood, they move on.
I did have one pair (they ALWAYS come in twos) inform me that they wanted to tell me The Truth. I responded thank you very much, I already HAVE The Truth; my Truth came way before their truth, and a crucial part of It was a Directive from G-d that It would ALWAYS be The Truth and never change, so whatever truth they thought they had, might be good for them, but it was false to me. They had no response to that and left.
It really is ebst not to get involved in conversations with missionaries. Unless you are Gavriel Aryeh Sanders or the like, you probably do not know enough about their “torah” to be able to refute their words. They know chapter and verse of Yeshayahu et al by heart. Most frum Jews do not, though if we open a sefer we could explain the meaning of the pesukim they misquote. But who wants to waste time doing that with them? I only care about JEWS not believing that tripe. The Goyim can believe what they will.
I once had a non-Jew argue with me about the meaning of the word “almah” which is their “proof” of the story of Mary and the “virgin” birth of her son. I explained that in Hebrew the word for a virgin is besulah, and that almah merely means a young woman (married or not) of a specific age. The person argued back with me that I was completely wrong, and I politely asked her not to tell me the definition of the words in my own Scripture, especially when she does not even speak the language, which I DO. Our rabbanim explained the meaning, and what THEY said it means, is what it means, and not what she needs it to mean in order to fit her theology. I learned then not to get involved with this type of argument. I don’t need to change their minds.