If your goal in a communication is to be polite, but you're not interested in what the other person is saying, the trick is to acknowledge "intent" and not "content".
Intent is the purpose behind a communication or behavior. So say to the Jehovah's Witness, "I appreciate your commitment to your faith to go door to door and spread the word. I'm sorry I don't have the time right now to give you what you deserve". By saying this you acknowledge the positive intent behind their behavior yet at the same time you are clear about your intent, you don't have time.
Alternatively, engage them in a lively discussion about the nature of reality and dominate the conversation until they are looking for excuses to excuse themselves.
Words by John Owen with Dr. Rick Brinkman, co-author of Dealing With People You Can't Stand, How to Bring Out the Best in People at Their Worst (McGraw-Hill), which is out now.