Contact Envato support and report the buyer by sending the screenshots of the conversations. Blackmailing is not tolerated at Envato and they will probably remove the comments/reviews and warn/block the buyer.
I have see users and yes it appears that this person jagochdu has made repeated comments across multiple pages of items belonging to @papon_it_solutions
The continued comments by the buyer should not be answered, but reported to Envato Support as @ki-themes has said. You are not going to win an argument with the buyer.