ANSWER:
It is not permissible to call a Muslim a kafir. The Prophet salla Allahu alaihi wa sallam said that whoever says to his brother: O Kafir, then one of them will take it (meaning one of them will be labeled as Kafir). Therefore, one must not say this unless the conditions are fulfilled and obstacles do not exist.
Besides, who cares whether they are in the fold of Islam or not except those who are exposed to their filth and one of their fans!