If the response rate wasn’t a little buggy, it’s intended function is to stay at 100% if you answer the first inbox or order message per user within 24 hours. It should ignore message that are pre-marked as spam and unanswerable, and if you report a message at spam it should ignore that too.
However, note the “IF” it wasn’t a little buggy. It is. It isn’t because it’s actually counting every single message. It just tends to go up and down on its own no matter what you do. Sometimes it goes down when you have no new messages at all. It also drops inaccurately if you get spam. The good news is that response rate has little or no actual effect on sellers that we can determine right now.
When it comes to spam, you can usually keep it from changing by replying to the spam with any response and then hitting report. Otherwise, if it goes below 80 or 90% and doesn’t come back up by itself within a week, you can always just ask Customer Support to reset it. They usually will. It also tends to come back up by itself over a little time, so I only ask CS about it if mine goes below 80% and “sticks.”