I strongly feel that anger/direct opposition is the exact wrong way to convince young people not to transition. They just interpret it like "YOUNG MAN GET BACK HERE THIS INSTANT!" or "you are NOT leaving the house in those shorts". Adolescent rebellion thrives off of that shit, and it just feeds their narcissistic victim complexes.
What you actually have to do is undercut their confidence. Don't be angry, just be pitying, confused, and-- more than anything else-- disgusted. It's not rebellion, it's wetting the bed. It's nasty, it's uncool, it makes people think they're ugly and gross, and you want to help them stop. Shame is the only thing that teaches people. And if you find out they're totally incapable of feeling shame, it's time to let them go, because they're not viable offspring.