Imaam Ibn Rajab (rahimahullaah) said:

[  أما أهل البدع والضلالة ومن تشبه بالعلماء وليس منهم فيجوز بيان جهلهم وإظهار عيوبهم تحذيرا من الاقتداء بهم ]

”As for ahlul bidah wad-dalaalah (the people of innovation and misguidance) and those who imitate the scholars, but are not from them, then it is permissible to clarify their ignorance and manifest their dificiencies, so as to warn (others) from following them.”

—————————————————————————————–

Source:

[الفرق بين النصيحة والتعبير : page 36] [The Difference between sincere advice and exposing faults]