@JoaNature: "Please tell me how to do it and review my response rate." Just so you know, even if Etsy Support or the Help Pages sent you here this is a seller peer to peer forum, and should not be considered a way to contact Etsy, if that is your hope (although Etsy does sometimes read and reply to posts).
The Star Seller metric for messages is very simplistic and binary - either you replied to (or marked as SPAM) that first message of a new incoming thread within 24 hours or you did not.
If the unanswered new thread is not a Help Request, at least for the time being, it appears that marking an unanswered new thread as SPAM (even after the 24 hour deadline has passed) will cause your reply percentage to change in a day or two (but not retroactively) if the original message was within the current star seller period. Note that subsequent messages from that account may go directly to the SPAM folder.
Unless something has changed recently, you can only mark a message as SPAM using the buyer app or a browser (even your mobile browser), the option is not available in the seller app.
