I have seen this scenario play out a few times where a new user kind of understand the idea and wants to participate, but doesn't understand/hasn't read the rules of the site yet. Because of this and their desire to participate they post a lot in a short period and while some of their posts are fine, others either break or don't quite follow the rules of the site.

How can I encourage a user to keep using the site while also saying at the same time some of his posts need to be revised or removed?