I live in a student house, and it was a student house for many years previous. I think they've learnt that we have absolutely NO surplus money - it all goes to tescos or over the bar. and I generally tell jehovah's witnesses I'm jewish. gets rid of them.
I love it when Jehovah's Witnesses come round. I engage them in discussion and refute them. Unfortunately, they tend to be a bit thick and not realise they've been refuted. Eventually they try to get away but I don't let them. It helps if you have a Bible to hand, since their Bibles are subtly rewritten, for some peculiar reason. They're obsessed with the bit about the sea in Revelation.
Try to reason with Jehovah's Witnesses. Score 1 point for each time they try to prove something by saying "but it says so in the Bible". Score 5 points if they agree that something in their manual isn't as accurate as it should be. Score 10 points if they want to leave plus get a bonus 5 points for getting a free Watchtower. I got 26 points once.