I've done the below:
Told them I am an atheist.
Told them I am an atheist, followed by "Would you like a cup of tea?"
They promptly left my doorstep.
Told them I am an atheist, followed by "I also burned a bible once."
Don't ask specifically what happened - suffice it to say that they left.
Told them I am a satanist, and that they just interrupted my weekly goat sacrifice.
I almost thought they left my doorstep for good that time.
I told them to gtfo before I called the police, and that I certainly would if they kept ignoring the fact that I am free to have any belief I want to. I told them that they obviously either ignored that fact, or simply didn't care, seeing as they apparently were content with continuin their attempts to convert me to the most twisted and bullshitty version of Christianity I have ever heard of.
"I don't want to deny my children a chance to celebrate Christmas, New Year's Eve, birthday parties, and all the other things you forbid," I told them. "So if you would please leave me alone before I drag you off to the police station for systematic harassment and ignoring my rights of freedom of choice, belief, and so on?"
I haven't seen them at my door since I told them that last thing.
So, the Witnisses of Jehovah were scared off by a 16-year old.