How can some sisters be convinced of the necessity of avoiding intermingling between men and women, when they believe that the hijab is sufficient for protection and allows them to mix with men?

The intermingling between men and women that is widespread today is an abominable and forbidden act, whether the woman is veiled or unveiled, because the hijab (definition) alone does not prevent temptation unless other regulations are met. It is permissible for a woman to speak to men or mix with them out of necessity, provided she adheres to the full hijab, including covering the face and hands, not speaking in a soft voice, lowering her gaze, avoiding forbidden seclusion, and refraining from shaking hands with men. We have explained the evidence for the prohibition of intermingling and the obligation to cover the face and hands. If you present this to them and they are not convinced, then there is nothing more expected of you than that, for the duty of the caller [to Islam] is only to clarify the truth and guide to it.