Respectful question for Jehovah's Witnesses...?

For three or four weeks now, there have been a few Witnesses coming to my house. Today, they came for the third time, and we sat and discussed a few things on my patio. While these people are very nice and I rather enjoy our discussions, I disagree profoundly with the Watchtower Society on a few key issues (Holy Trinity, Dual Nature of Christ... all the things you would probably expect me to disagree with as a Lutheran). We haven't begun to discuss these things yet, but I would imagine they're working up to it. Like I said, I enjoy our discussions, but I already have my own beliefs. I feel very strongly about those beliefs, and because of that, I'm not at all likely to convert. My question is this: Would it be more polite to tell these Witnesses that I'm not interested now, so that they don't waste time and gas money coming out to my house, or should I continue to invite them to sit and discuss things with me until we get to the debate about the aforementioned doctrines? Thanks.