I think you could be right Apog, whenever I have anyone come to my door, either a charity or selling something I say I don't want to talk at the door but do they have a website. This gives me the option of looking at what they are all about without feeling hounded or under obligation. If I want to go further, ie make a donation to a charity or buy something I can do it without any pressure.
I think this will point people in their direction at first, whether it'll make them want to be called on is another matter but I still think it's quite an effective ploy.