On wifi a teenager is as easily blocked as anyone if you control your router and don't share the password, just block the site in question and you are done.
Most of us hold the purse-strings on our kids mobile subscriptions, credit card use, and control the mailbox keys in their house. Any help with this stuff just ask.
If they can get through all of that you have something functionally equivalent to an adult on your hands.
We've gone beyond security now and the remaining options are surveillance and parenting. I'll leave this dirty work to you all :)