I've found the best way to deal with them is to answer the door in your boxers, but make sure you're "peeking" out the front.
Act like you don't have any idea you're swinging free and be real enthusiastic about talking with them right there at the door.
The couple Mormon guys that ride all around here on their bicycles knocking on doors won't even turn down my block anymore.
See, a lot of the most fervent Mormons are homosexual. There's a lot to it, but their church is worse than the Catholics when it comes to the whole "naughty elders touching people" thing but they're a hell of a lot better at covering it up. Probably because they're smaller and way more indoctrinated.
Anyway, it's a major, major no-no to be gay in the Mormon church and they believe big-time in the whole "ex-gay" BS. So the few gay guys that they've got preach and pray extra hard to try to make themselves more straight.
Even better, the Mormon church keeps tabs on them, watching them like hawks, and they just love to "train" them by sticking them with another guy they know is straight and sending him out to knock on doors and such.
So you answer the door with a swinging cock and you've got a really good chance that one of the meat-gazers is really hating that he doesn't hate the view, and the other is trying to figure out whether the first one is still gay... it's hilarious.
The straight guy doesn't want anything to do with you, and the gay one can't risk sprouting wood in front of the straight one.
They won't come back.