You can tag posts. with pluses and minuses.
But that would lead to anarchy & chaos you probably don't want. For instance +"troll"- tag...<p>Upvoting can be manipulated with relative ease.
But if that happens then you will probably restrict the access, which would effectively kill the fun. You could devise a system for upvote justification though. A short clickable tag or "essay" why the upvote is "justified." The same with extreme downvotes.
Take care