The Question: How do I make it clear to religious groups that occasionally visit that I'm not interested, without sounding rude?
The Story: On Wednesday night, as I was doing homework, I heard a knock on the door. Not knowing who it could be, as it was 7 in the evening, I answered the door. I was expecting a relative but it turned out to be two young women from the Church of Jesus Christ of Latter Day Saints. As I was really busy at the moment, I quickly wanted to say "Sorry, I'm not interested" but my problem is that naturally, to everyone in the world, I do not seem content. Growing up, people would always tell me to "smile, life is too short to be grumpy all the time". The point here is that I didn't want to "politely" inform them I'm not interested as most people would think I am very rude to too my face and voice. I simply don't want to seem like a rude person. It also didn't help that they were really cute.
Anyway, we ended up "chatting" for a good half hour and with me trying not to come across as a dick, one thing led to another, and suddenly now they will be visiting Monday night again. I was sort of interested in what they had to say as someday I would like to join a religious, but right now, will university applications coming up, I don't have the time, and won't for many years to come. Anyway, as mentioned above, how do I make it clear that I'm not interested at the moment, without coming across as an asshole, and also ensure they won't come back.
Additional info: I'm extremely self conscience, if you haven't noticed.
EDIT: come knocking*