I can't see any obvious way to turn off the BT with Fon on the router set up/software.
Put a timer on the power supply, and lock it in a cupboard....that will screw him up...
How about setting up parental controls on your BT account page?
Log into your bt account online and activate parental controls, that should also work when using bt wifi with fon
Take the bloody phone off him at 21:30 and don't let him have it back till 07:00 next morning.
Simple