In this situation, you can approach the police or cyber cells with a criminal complaint.
As per Section 66D of the Information Technology Act, anyone who cheats other people on Whatsapp by pretending to be someone else can be legally punished. The punishment is imprisonment for up to three years and a fine of up to Rupees one lakh.
Further, under Section 354A of the Indian Penal Code, any man who makes sexually coloured remarks is punishable for the offence of sexual harassment. The punishment for this is imprisonment for up to one year and/or a fine.
A criminal complaint can be filed by the victim of a crime, anyone who knows the victim, or someone who knows that a crime has taken place. To know more about how to file the complaint, read here.