Preventing Funtions to EXECUTE

How can I prevent a function, with javascript, to not execut… or making it null…

I can do this?

function hello(){ alert(“hello”); }

hello=null;

hello(); //So the alert don’t shows up

I said this bicouse… I whant to remov the adds of a freeserver where I have some files… That adds ar put in my html with a window.onLoad kind of thing… If I know the names of some of that functions… ther’s a way for nulling them?? so the add never shows up?