Some users have legitimate purpose for making multiple posts. Sometimes a best thing a user can do is to make a new post. But there are some newbies aren't aware of rules and that's why they make multiple posts.
So, what about displaying a warning message like this?

Warning: The last post of this topic has been made by you.
Unless you are sure making a new post is needed, please consider editing the previous post.
If you are quoting multiple posts, please quote all of them in a single post.

I am sure this warning can reduce multiple posts.