This may be weird because somehow upgraded users will have the right to limit newborn accounts's posts but i'll explain it just down below.
We all know there is no actual way to stop leechers from joining the forum at all, but we can hopefully limit it. For example users with X minimum requirements can submit for a staff manual verification to get the permission to limit a new user posts if he's just spamming around until a staff member get online to manually review it. And if an abuse is detected he'll have to face the consequences of course.
Min requirements example: -Upgrade -Active member rank