But if you really want to just turn it off, and on for you. "Allocating" wifi resources can be done with the expensive routers like cisco and the likes. But those are pretty expensive ones.
Its going to depend on your router. Many routers have parental controls. This allows you to turn on and off particular IP addresses or mac addresses. So they can technically connect to the router but it will not let them pass though to the internet. Many of these can also limit what site they can access....so you force them to only go to educational sites you choose for example.
Now if you have a very simple router the changing the password method stated above works on any router but is a pain in the butt since it also requires changing this in the end PC all the time.