One, it's libel, not slander. Two, truth is a valid legal defense for libel. Three, an opinion is not libelous. However, threatening extortion actually is illegal, and I read they just passed new cyber harassment laws making it a federal crime to use the internet to harass someone. Best to just let it gooooooo.
“When plunder becomes a way of life for a group of men in a society, over the course of time they create for themselves a legal system that authorizes it and a moral code that glorifies it." ~Frederic Bastiat