The best way is, assuming you can find all the devices they have access to, is to remove your teenagers devices until they learn the house rules. This will of course undoubtedly cause tantrums and melt downs, on both sides.
You will never control Internet access with technology. There will always be a way around it which most computer savy kids will find and most of which are relatively simple to implement.
MAC address filtering can be problematic with a lot of devices because they use MAC Randomisation and it will require a lot more effort to block the device which can be overcome with a factory reset of the hub.
Turning off Wifi as you are aware still leaves Ethernet access. This can be circumvented by using an Ethernet connected device such as a PC to become a wireless hotspot which will then allow wireless devices to connect to the Internet.
Changing router or adding wireless access points with a different SSid /name will not solve the problem either because there will be ways around what ever it offers.
Turning off the wireless signal
See link
How do I turn wi-fi on and off on the BT Hub? | BT Help
may work for as long as your hubs Admin password is not known to the kids and as long as your MyBT username and password is not known because they can use that to access the hub and reset it which will turn the wifi back on and of course you need to guard the Smarthub to prevent it being reset.
Good luck. Please keep us updated because if you do find a solution I am sure there are millions of parents who want to know how it was done.
