How to politly decline the evangelists?

Do not be any more polite to them than they are to you. Treat them just as you would anybody else who is selling something and won't take no for an answer. Because that is exactly what they are doing. If you seem like you might be persuaded they will make note of that and they will be back. I am normally very sunny and nice, but firm. I wish them a great day, comment on the weather and offer a bottle of water to take with them as they go their way.