Same as for posts, build capability to vote on comments as well.
Don't allow the same user to vote multiple times on the same comment/post. Hint: you could use validations to do this, but you'll need to 'scope' the validation. Need to play with it and read documentation. There are other ways to tackle as well.
Prevent voting if user is not authenticated, also don't show the voting arrows if not logged in. The former requires doing something in the controller/action, the latter requires something in the view templates.