I've often wondered about that myself, Simons Mith. After all, if my forebears are to be believed, even the tramps used have a 'secret' code which they scribed before someone's dwelling to indicate 'Probable', 'Possible', 'Unlikely' and 'Don't go in there the dog will eat your balls and dump your remains in the bin'. Then again, most tramps are probably better organised (and intelligent?) than those of whom we speak.
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.