Sorry OP, YDI. If all you did was shut off the Wi-Fi then you're a bit of a dick. If there's nothing better to be doing (like if they're not neglecting chores) then shutting off the Wi-Fi is just being controlling. My dad used to shut off the Internet if he thought I was spending too much time online, but the only other alternative available was TV. He would even shut it off after dinner to force me and my brother to watch TV with him even though we weren't allowed to talk at all during the shows and he would go outside to smoke during commercials. It was controlling for the sake of controlling.
Shutting off one screen for another is pointless. There are many things to do online that encompass a bunch of other offline activities, such as reading news articles, watching movies, learning how to do stuff, connecting with friends, reading books, listening to music, doing homework, playing games... It sounds like if OP's kid was doing these things offline, such as in their room with a Cd player, magazines, books, game consoles, and a cellphone, they'd be happy that their kid was doing so much, but because it's being accessed with a computer, you assume that nothing else is going on and that they're only doing one thing.
My question is what did you want them to do once the Internet was off? If the answer is just a general "anything but the computer", then you have a lot to learn about the things that are available to do online. If you have a problem with Internet use but are happy to sit as an inactive participant in front of the TV, then YDI.