I have asked people "Could you please park on the street?" "Pardon me, there are kids riding their bikes on this street, could you park on the street?" "Hey, that isn't a parking spot" left a note. I've been told "fuck off" "mind your own business" or just looked at like an ugly thing and ignored. Twice I've had it seem like it might get violent, one of those times with my one year old daughter in my arms. I wasn't always perfectly polite, and I have after being told to fuck myself directed some choice invectives in return.
I would say anyone who blocks a sidewalk with their car doesn't really give a fuck about being good to their fellow man. They either don't really think of their fellow man (probably the people who act like they don't hear me or don't understand what I'm saying) or they are just anti-social pricks who only care about themselves and don't give a fuck to inconvenience others (the ones who want to get in my face about it).
I have gotten neighbors on my street to not block sidewalks, but I don't think they actually understand why they shouldn't do it, or why I care. It's as if thinking about the consequences of their actions when there is not personal downside to what they are doing is unimaginable. I live in a duplex. My next door neighbor has relatives who drive out in two minivans a few times a year. If we aren't parked in the driveway when they get here they park both of their minivans in the drive with the back one blocking the sidewalk. On THREE DIFFERENT OCCASIONS I have knocked on the door and asked them to please move one of their vans to the street. They look at me like I'm a dick or a fool shaking their head every time they see me. (I'm super nice about it, I like my neighbor, but not her relatives). I'm sure I'll be asking them to move their goddamn van again, I don't really understand why I have to keep doing that.
The other way society has for getting people to not block sidewalks is an expensive ticket, I would actually say that my method is probably a bit more gentle.