I talked to some Mormons that came to my door once. I can't remember what we talked about, but when they said they'd come back later, I told them I didn't live there. I alternate between my parents' houses, so I legitimately might not have been there if they came back.
If you can't contact them, I would just tell them politely when they visit that you're not interested in soliciting anymore. They should understand since they just said they would come back without asking you.
