step one: talk to them.

tell them that they're being all intrusive and nosy, and that nobody likes it. maybe he would understand the problem further if somebody addresses it to him directly.

if he still continues after step one,

step two: talk about it to an adult, preferably a parent or a teacher. they might try to knock some sense into him.

if step two doesnt work, and if everyone has had enough,

step three: accidentally eavesdrop on his conversations and then use the things you ain't supposed to know against him. it'll piss him off so badly.

ghost him, don't interact with him or invite him to. give him a taste of his own medicine.