Can you stop teenagers looking at online porn?Err, no. It's not a "dilemma". Dilemmas have to have (at least) two rational options. If one of them is silly, no matter how unattractive, then it is merely superficial. Next, please.
Oh, sorry, you wanted a little more relevant comment? The fact that you weren't allowed to be sold porn mags (or cigarettes) until you were whatever-age-it-was back in ancient days (16 for the fags) didn't stop teenagers getting hold of them. Magazines and evil-cancer-sticks are easier to control than digital pictures.
You couldn't even attempt it without placing hideous restrictions on those of us who are legally allowed (although uxorially disapproved) to see porn. So just don't.