More than a hundred islamic scholars and clerics have spoken out against the IS and basically declared them apostate. This should serve to rally a lot of opposition against the IS. And shut up others.
https://news.vice.com/article/muslim-scholars-make-the-theological-case-against-the-islamic-state
https://news.vice.com/article/muslim-scholars-make-the-theological-case-against-the-islamic-state