Sayeed Husayn Shahrudi said in his fatwa:

 وقد دلّت الرواية الصحيحة على التخصيص فان الافتراء على المبتدع ليس حراماً لمصلحة أهم، فان الشارع يريد ان يبتعد الناس عنه وعن أفكاره المنحرفة بأي صورة ممكنة.
The authentic report states that one can fabricate lies upon innovators specifically and that it isn’t unlawful for there is a great purpose behind it. For the Law-maker wants the people to stay away from him (the innovator) and his strange ideas, no matter what the cost.

 

Don’t forget almost similar fatwa from al-Khui, which is on our site, and hadith which they has ascribed to their Imam.