I've read several discussions on a couple of different homeschooling message boards recently about what to do when missionaries come knock on your door. I've seen quite a range of reactions, but they are generally quite negative and I am surprised at some of the nasty things people will suggest saying or doing to get rid of whomever is at the door.
I firmly believe that if a person is not interested in what a missionary has to say, she should tell him and close the door. There is no obligation to listen to something you're not interested in, especially on your own doorstep.
But I don't think it's necessary to be rude. It's not necessary to argue. Simply end the conversation immediately. Those missionaries are quite probably having a hard time and you don't need to add to it. And maybe, just maybe, you could give them a glass of water or a friendly word before sending them on their way.