Do a search for your favorites (fav:AM-sama), then enter this for your URL:
javascript:for (i in (thumbs = document.getElementById("post-list").getElementsByTagName("span"))) if (/thumb/.test(thumbs.getAttribute("class"))) Favorite.destroy(thumbs.id.replace(/p/,''));Only does one page at a time, untested (of course), and might miss some if Danbooru can't keep up with that many pending requests at once.Make the "javascript" part a bookmark, stick it on your bookmarks toolbar (assuming FF), and alternate between clicking that and refreshing after the pending requests go through.
Updated