Find And Replace All Instances Of A Word In The Page?

I am looking for a script that will find and replace all instances of a word in the page. how ever there might be more than one instance. I'm not that crash hot on javascript and all i have at the moment is:

This script should look for UNASSIGNED and replace it with nothing

HTML Code:
function func1() {
var str=document.getElementById("UNASSIGNED");
document.write(str.replace(/UNASSIGNED/g, ""));
}
window.onload=func1;

Also wanting another script that searches for a string via ID. if the id has SP.XXX <-- XXX being random numbers then i want it to find the and replace this repeating occurrence with a