Dear Clipboard Guy:
First, let me say that the use of "dear" is merely a convention for a letter greeting, rather than an expression of affection. A colon may seem formal, but I'd like to remind you that you're not a friend or family member or even an acquaintance I think I might kind of like one day; the reason you don't get a friendly letter with a comma after the greeting is that you're a stranger.
Your status as a stranger might have been driven home to you when you walked up to my house. Did you hear my dog barking from the moment you stepped on the grass? Remember how her barking increased in both intensity and volume when you stood at the front door? How about when she jumped up on the armchair and barked at you through the window?
What I didn't have the time or inclination to tell you:
1. I've been regretting opening doors to clipboard-carrying strangers for thirty years, which experience assures me that there is nothing you can have to say that would interest me, partly because I have no interest in knowing someone who invades people's privacy for a living, but mostly because I know you wanted something from me that I didn't want to give you: money or a signature.
If I had opened the door, I know I would have been sorry. You might have been sorry, too; I would not have bought what you were selling; I would not have signed your petition. I never do. I don't want strangers appearing on my doorstep to sell me things or to proffer petitions, and therefore I never reward the unwanted behavior.
2. In order to open the door to you, I would have had to grab my dog's collar and drag her, toenails scraping on the wood floor, into another room, and lock her up. I did the cost/benefits analysis and decided not to bother. My dog is getting on in years. She has hip dysplasia and arthritis. Being yanked around the house would only aggravate the pain she is already in: you might or might not agree that's an excessive cost for the experience of declining whatever you're selling. It doesn't matter either way; I care a lot more about my dog's feelings than I do about yours.
3. When you knocked on the door, I was working. I don't show up at your work and bother you. I wish you would show me the same courtesy.
4. When a person asks you THREE TIMES--politely, in a neutral, civil tone and without brandishing a weapon--to vacate the premises, the sensible action is to shake the dust from your feet and hie thee hither, not to remain on the doorstep making facial expressions and gestures indicating your chagrin at being insulted. If we lived in Florida and if I owned a gun and if I had any whiff of a hint of a thought that I might be under threat, I guess I could have shot you.
If you believe that we all of us who are not you have some obligation to do exactly what you would like us to do, e.g., open the door, buy your magazine subscription, do your laundry, put up with your nonsense, and so on, much disappointment awaits you. To be plain, just because you want me to do something doesn't in any way obligate me to do that thing. (Although I very much hope you will learn from this experience and stay away, so I can't imagine what else you might want from me.)
Best regards, (which you should interpret as a meaningless convention)