how do i switch off wifi on router NETGEAR DG834G?

i have a netgear router model DG834G fitted to my desktop. we bought this to run my sons laptop on wifi. Everything works fine except for him turning it off for bed time. he is 14yrs old and messes about on it sneakly until about 11pm. Due to this his school work is suffering and it will cause teenage attitude if i take it away from him.
I need to know how to switch the wifi off from the router or settings at a resonable time at night. i was told there should be a switch on router but there isn`t.

Please help me.