The Chofetz Chaim writes [כלל ח' ה ז] that it is not only permitted to say Lashon Hara about a person who denies the basic tenets of our faith but it is actually a mitzva. In addition - if a person is מוחזק כרשע - is an established evil doer [he does aveiros on purpose] then it is also permitted [and presumably a mitzva] to speak Lashon Hara about him.
If someone is within the pale a bad word my not be said. But those outside the pale are a different story.
