You're going about it the wrong way. By turning off their wifi you're simply presenting them with a challenge to get around your block. Just tell them no computer between 10 and 7, or whenever, and be done with it. If you see them on between those hours, easy to do by looking at your network map, they get their computer privileges taken away for a week or whatever. It's called parenting, not policing.
...the secret is to bang the rocks together, guys.
That which does not kill you has made a tactical error. --Tayler