prevent him/her from doing it
Je resterai éveillé pour l'en empêcher.
I will stay awake to prevent him.
I have to stop him.
Tu n'aurais rien pu faire pour l'en empêcher.
There was nothing you could have done to stop it.
— Tár (2022)
Mais l'en empêcher est inutile.
But there's no point in trying to stop them.
— Perfetti sconosciuti (2016)
I wanted to stop him.
We must stop him.
I should have stopped him.
You have to stop him.