أَغَيْرَ اللَّهِ تَدْعُونَ
Do you call upon someone other than Allah? (al-Anʿām 6:40)
This functions as denunciation or rejection (inkār). It does not suggest that the speaker lacks information about what the addressees do; it condemns the conduct.
فَهَلْ أَنْتُمْ مُنْتَهُونَ
Will you then desist? (al-Māʾidah 5:91)
The interrogation carries the force of a command: stop the action. Question form makes the direction forceful and immediate.
The distinction is that rejection targets the reprehensible character of an action, whereas command directly urges the addressee toward what must be done.