Is it ALL visits you want to stop, or are you okay if they only come when they're doing the street?
First one: You could ring, but I'm pretty sure they'll send a brother out to speak to you, mainly to make sure its not because you've had a very bad experience. From then on, 2 brothers will visit you once a year to see if you would like us to start visiting again or not.
Second one: If you just want the one-on-one visits to stop, you're going to have to tell the people who are regularly visiting. Don't worry, they won't be upset, it'll be fine! They would be more upset to find out they've been 'pushing' literature on you when you didn't want it!
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