Anyone can change their posting habits if they are committed to doing so. The only limitation is that most spammers have account farms operating multiple profiles at the same time, which makes it difficult to churn out quality content.
Limiting the accounts you post on and putting some effort into the conversations you join on the forum is a sure step to making anyone's posts constructive. After a while, the user can send a message to those who left the feedback asking them to review their post history.
I agree with this. Someone using only one account wouldn't have a hard time improving the quality of their posts unless language is a barrier for them but even if that is the case, I believe they should be able to write quality posts in at least their local board. Besides, learning English is not that difficult even for someone who has never tried it before. Some tools and apps can help you learn the language.
However, this is only possible if they acknowledge the fact that they weren't doing things the right way and they should be determined to learn. Many spammers are too lazy to learn stuff and you can't become a constructive poster if you don't have enough knowledge about the things being discussed.
Overall, in my opinion, it's possible for a spammer to become a good poster but it's not going to be easy for them, if they are ready to work on it, it's possible.