What steps should I take if my builder refuses refund?

I booked a flat in August 2018, expecting the handover by March 2021, as per our agreement. Despite three years passing, the builder has not provided a handover date. I submitted a cancellation request, as suggested by their CRM team. Over the last 10 months, I’ve persistently reached out to the builder through phone calls, SMS, WhatsApp chats, calls, emails, and their CRM portal, seeking a refund. However, every 15th of the month, they make false promises of a refund, only to ignore my attempts to contact them. According to our agreement, a full refund with interest should’ve been provided within 15 days of cancellation, but they’re not adhering to this. Moreover, they have blocked my phone number and are unresponsive to my emails. I seek advice on addressing this mental harassment, obtaining a refund of the advance amount of ₹7,00,000, and pursuing justice in this situation.