How does Ender “forestall [the] vengeance” of Stilson and his gang in Ender's Game?
To forestall vengeance, Ender attacks Stilson while he is lying on the ground. This way the others will know that he is an animal, lying in wait to get them when they are not looking.
In school, Ender knows he will be bullied because he is a third child, and families are only allowed to have two. In order to prevent the boys from bullying him, he decides to viciously attack one of them, Stilson, in order to scare the other boys so they leave him alone.
"…But just remember what I do to people who try to hurt me. From then on you'd be wondering when I'd get you, and how bad it would be." (ch 1)
Since the boys do not follow him, Ender knows he is successful. They have decided that he is an animal, because in warfare you do not attack an enemy when he is down. Even at six years old Ender knows this. He is already separating himself from the pack. The adults always knew he was special, but now the other boys in his class do as well.
Ender tries to be tough, and warns the boys that he can do worse to them when they are not looking. In reality, he is just a scared little boy who is worried about what the others will do to him and is trying to protect himself.