Zest Money - Cash back

I bought a laptop last year December from Croma in which a Zest Money's employee offered me the discount on purchase, like a cashback return after * months of Rs**** and monthly interest back. Offer was good so I bought it.
Paid * installments and none of the cash backs were received by me I made a call to Zest Money customer care number told everything they agreed to it that yes I'm yet to receive both the cashbacks, they gave me some time and promised that I will receive it but those were false promises made multiple calls after that and then they started saying no that I'm not eligible for the cash back. I even made advance payments but still no resolution provided multiple calls were received and I have to tell the same story a thousand times still they didn't mention it (my case) on my profile. A few months back they sent a person at my house for demanding the last emi which I didn't pay because the cash back was not received. I told him the same store even after telling it on the calls a multiple times, he promised he'll provide the solution but then again the same thing happened none provided.
Now I'm receiving threatening and messages for defamation. They have started mentally harassing me on whatsapp messages and when I call them my number is blocked and if I ask them to call me back they just leave my message on seen.