Not an option in either SMessages or Gmessages that I'm aware of. The intended behavior is to show recent conversations so that you have the larger context of the conversation you are in as well as the ability to easily reference previous conversations.
Not exactly what you are asking for, but the only way I know of to reply to messages without seeing the entire thread or conversation list is to reply directly from the notification. This prevents you from needing to open the app and see your various message threads.
To do this, you can pull down or click the arrow on the banner to expand the notification and the click "reply." You can also make the reply option appear on the banner by enabling detailed notifications.
If you do not need to access your previous/unread conversations, you can archive messages to clean up the appearance of your inbox.
