I wouldn't look down on the couple as 'nosy.' If some guy is parking on a residential street for a bit every day for months and doesn't even get out of his car I'd be a little less comfortable than I was before he started doing it (granted circumstances of his parking there were unknown). And if the couple was concerned about this man, I mean calling the cops is safer than approaching him. Boxing him in is not though aha.
They went about it terribly, I assume it was building up in their minds for a long time. The husband intrepidly telling his wife "This is the day honey, I'm gonna go out there and tell him to beat it!" "No, let's box him in like we planned!" Funny story, but I'm not exactly rooting for the guy to keep parking there so that couple can get their comeuppance, there's essentially nothing on the line in my mind.