Don't you think that it could be good to explain to any new user, at least the first time he or she connects, what are the rules of the site concerning : accepting answers, upvotes, downvotes, ...
Perhaps the best thing you can do to turn first-time users into valuable contributors is to encourage their first efforts. This is also the best moment to educate new users on the distinctive features of your community. The site's About page was designed to do just that. There's a handy way to link to that in the comments:
Have you seen our [about] page?
Sometimes you might want to prompt a user to edit their post either because it's poorly formatted (though feel free to fix such problems yourself) or is unclear. Again, it's easy to provide a link in the comments:
Would you mind making an ?
When someone fails miserably to ask or answer a question, you could politely include a link to How to Ask with
[ask] and How to Answer with
[answer]. To point to this meta site, use
[meta]. (For a complete list of shorthand, see the comment formatting help.)
Not everyone who posts on the site will be a solid contributor in the future. But you can increase the odds by being part of the site's welcoming committee. There's even a review queue for just that purpose.