I will never buy anything from a door to door salesman based on principle alone, even if I need it - I'll go to someplace else and get it from them. I had a guy last year come by and try to sell me a security system and when I declined he said something to the effect of "so I guess you don't care about your wife and kids." He got a stern "fuck off" and the door slammed in his face.

I had another guy (again trying to sell a security system - what is it with those guys?) stick his foot in my door when I opened it so I couldn't close it. Very intrusive.

Pet peeve of mine, I guess, but I can't stand door to door salesmen.

Edit - or mormon/jehovah's witness missionaries, but at least they're trying to advance something they care about. I know a guy who uses a couple of mormon kids as free labor for home improvement projects - and he has no intention of converting...