1. Psycho killer. We've all seen the movies where a contract killer or other lunatic is looking to kill someone in the neighborhood, so he needs a safe, comfy hideout while waiting for the opportunity. That comfy hideout is your house. When you open the door, you get plugged so he can watch for the guy next door who's in Witness Protection or whatever. Why would anyone invite this kind of unpleasantness into their home?
2. Religious zealot. This deviant is trying to earn a place in heaven by inflicting his irrational beliefs on you. No, thanks.
3. Politician or politician's deluded acolyte. He wants your vote so he can spend the coming decades earning a high salary and rewarding his supporters with your tax money. Get off my porch before I release the dogs.
4. Girl Scout. Selling diabetes for $4 a box, and you feel like a jerk if you say no. Shouldn't that kid be in school?
5. Neighbor. Wants to suck you into his nonsense, perhaps by hosting a Neighborhood Watch meeting or taking care of his dog while he's on vacation. Sorry, I'll be out of town that day/week/year/decade.