We had JW showing up occasionally back in the day.
From a very young age, I was extremely anti anything that tried to overtly persuade or manipulate me into doing or thinking anything other than I felt like. This would include sales people and any type of religious cult.
So, one day, they came again.
I remember that day, because my mother was always being harassed by people like that - as she's as close to a pure "altruist" as you can possibly imagine. She just dreaded when JW would come and bother us. I just remember her reaction when the doorbell rang and we both knew it was JW again - as it was a sunday, IIRC.
This time, I opened the door. I must have been 14 or 15 years old. They started with their usual "do you want to save the world" spiel.
Then, for some reason, "something" took me over - and I became VERY direct. Not physically aggressive - I just adopted a tone of voice and a way of conveying my message that was impossible to side step or talk your way around.
I essentially told them that I know they walk around with their little notebooks - and that they should write down that this particular house should never be visited again, and they should make sure everyone else in their "flock" got the same message. They tried to object - but I was rather firm in my attitude.
That's about it, really. We never saw them again
I use a similar strategy when I get cold called by whatever telemarketer. They ask "Hi, do you have a moment to .." - and I simply answer, in a very calm but extremely firm voice, something along the lines of "I have no interest and I never will have any interest. Don't call me again."
Surprisingly, it works.
Salesmen and people who're used to manipulating people rely on them being polite and pleasant - because that's how we're generally conditioned. It's very, very difficult for the average human being to be openly impolite or unpleasant to a complete stranger with a great big smile on his or her face - even if it's just a few words that honestly and simply convey what we're really thinking.
So, while it may feel like you're being cruel or overly aggressive - it really is the best way to avoid these things.
It can take some conditioning - but I highly recommend it, unless you enjoy other people stealing your time exclusively for the benefit of someone other than you.