However, there is no point in stopping spammers to rank up especially if they too created quality posts.
Spammers don't create a single quality posts. They are busy with spam and don't mind, don't spend a second to think for what they will write.
If a spammer changes from spamming to creating good quality posts, that member deserves to get merit for good posts, not for spam posts certainly. If a spammer suddenly makes a long post, you can use google search to find for plagiarism. There are not many spammers change and write long posts. They copy and paste.