Have a look at the connected devices (on the router's config page, probably http://192.168.1.1) and try and spot the Wii (obviously it has to be connected first) and write down the MAC address or something, then there should be an option to deny access to certain devices, and you just punch in what you wrote down. It all depends on your router model, though. Would still be a good idea to have a look at some software that does it easily for you.