The moral and legal issues surrounding unprotected wireless networks are essentially irresolveable; there are valid arguments in both directions, and there's no way to reach an agreement since different points are more important for different people.
That said, I'm assuming that carbonmotion is explaining his particular situation accurately, in which case it'd be hard to make a strong argument that the kid isn't doing something wrong, if not in principle at least in the spirit of what he's doing.
The name of the access point implies it's not for open use, the kid apparently knows whose point it is and hasn't asked permission to use it despite having ample opportunity to do so, and his excessive use is causing a measureable negative impact on the owner of the connection. Besides, if he's downloading porn it might well be illegal use as well, potentially putting carbonmotion in some legal jepordy as well. (And if you want to get into another huge grey area, he could also be making carbonmotion 'responsible' for providing adult material to a minor.)
Point is, the kid's using the connection in bad faith; he knows whose it is, he knows that it isn't intended to be open, and he's using it enough to cause the owner an inconvienence.
This doesn't make him some sort of evil person, but playing a prank or two on him before cutting him off isn't exactly a high crime either. Although getting him in trouble with his parents, which he apparently deserves to be in, seems a much more appropriate solution.
By the way, for those saying he doesn't deserve to be pranked a bit because it's the victim's fault, isn't it just as legit the other way around? I'm sitting there minding my own business, and there's a bunch of traffic flowing through a router on my property, that I own. I'd say I have the right to intercept, mess with, or alter that data if I feel like it. If my open network hanging around in his living room allows him to use it, then his open data stream hanging aound in my router allows me to use that, too. His fault for not securing his connection, and his computer, right?
