Userscripts.org

I actually already wrote a script for removing the grooveshark ad before I found this. Since yours doesn't work I'll post mine.

// ==UserScript==
// @name           Grooveshark ad remover
// @author         KnifeySpooney
// @version        1.0
// @description    Removes ads from Grooveshark
// @namespace      http://itsjareds.scienceontheweb.net/
// @include        http://listen.grooveshark.com/*
// ==/UserScript==

var adBar = document.getElementById("adBar");
var mainContent = document.getElementById("mainContentWrapper");

adBar.style.display="none";
mainContent.style.marginRight="0px";

Yep, works great for me too!