Jehova's Witnesses keep knocking on our door.

Although I think they're not just idiots, they actually dangerous idiots because they pro-actively go out to corrupt vulnerable people, filling their heads with hocus pocus and jibberish, I usually politely tell them that they're barking up the wrong tree and just shut the door.

But new ones keep coming back, time and time again.

Does anyone have any ideas for getting rid of them once and for all (other than by joining them - not a very viable option)?