Try to get an 18 yr old, or a 21 yr old to stop watching it and you won't. They will have watched so much porn by then that they care more about the porn than the women. They need to learn how harmful it is before they are addicted
That's the thing, isn't it?
It's normal for teens to be interested in sex. It's normal for them to want to search out erotic material (how many of us trawled our parents' bookshelves back in the day looking for the "naughty bits" in novels?)
What you want to do is get across the idea that violent, brutal sex isn't normal. And that teens have a choice.
To put not too fine a point on it, if you watch brutal, violent porn and wank to it, you set up all sorts of pathways in the brain such that this rapidly becomes the only sort of sexual scenario you can get aroused by. This screws up the possibility of normal, consensual sex, and screws up the women you come into contact with.
I don't want this for my son. The nightmare is that in trying to explain this to a child before porn has done the damage, you feel like you are yourself damaging them, taking away their innocence.
The trouble is that you don't have the choice not to, because the porn industry is going to take away their innocence anyway.