International froud against export order received

Dear all, I am doing small export business in Delhi, I got a order from one client from Austria, they bought product from me 3 times ,after receiving product, they told me they don't like product and send different product, but still I was replaced once product. They have order for synthetic diamond but after receiving product they told ,they want real diamond instead of synthetic diamond otherwise they never return product and they want refund. They already initiate chargeback and they complain many where in India and start review in different place like google and all and also calling from unknown no and they told they have contact with criminal In all over world. He actually want to destroy my business, kindly guide me what should I do? I have already loss lot if money