I understand the neighbor's thoughts in this case. He pays $ to have that extra spot, even though it is against the lease, and visitors are not supposed to park in resident spots (without being registered, I assume, as is the case with most places with assigned parking).
On Kevin's side, he's never been there before, the numbers/markings are hard to see, and there were apparently visitor spaces open when the neighbor came home.
In this case, I would have been peeved, but would have at the most left a note on Kevin's windshield informing him that yes, he was in a resident spot, and please don't park there again. I would have also recorded his license plate number... and parked in a guest spot. If that same car was there when I came home again the next day, THEN I would call management to get them involved - mainly because I wouldn't know what company they have contracted for the lot. Most apartments have contracts with specific companies. I wouldn't bother going door to door because who even knows WHAT door that car belongs to? It doesn't matter - I put notice on the windshield, they're still there (or they're back again) - out they go. Not my problem anymore.
The neighbor committed two illegal things and one etiquette-inappropriate thing: first, illegally leasing the spot against the apartment lease; second, double-parking to block Kevin's car in. The etiquette thing was seeing the note, refusing to accept it, and waiting until DH came to apologize face to face before he moved his vehicle so Kevin's car could actually leave it. I fail to see how this actually helps the neighbor at all - it just prolongs the problem. Scenario a: you come home, you see a car in your spot. You double-park it in and go to bed. Your wife leaves at 8 am the next day. You park your car in the now-vacated spot... Kevin leaves. Your wife comes home and the spot is open so she parks in that. Total time your car is illegally parked: 12 hours or so? Scenario B: You come home, you see a car in your spot. You double park it and go to bed. Your wife leaves at 8 am the next day. You do not move your car. Someone comes to the door and apologizes - you throw a fit, move the car finally. Total time your car is illegally parked - more than 12 hours. Scenario C: All of the above, except you do not move the car. DH is forced to tow the car. You're out $$. Scenario D: You come home, car is in your spot. You park in a visitor space and go to bed. Your wife goes to work, and you move your car to that spot. Kevin leaves. Your wife comes home and parks in your other space, which is now empty. Problem solved - no need to write notes, fret, complain, get towed, get worked up...