Cap the number of upvotes that a user gets each day and give explicit feedback on how many upvotes that they have left.
I always think of the Perlmonk progression as a good idea. It will require a radical departure in use/interaction and might be seen as too controlled. PM had the advantage of doing this from the start (as far as I know) PM #244776
It certainly works. I always go there ready to really think twice before I post & you learn. HN appears to be more OpEd a lot of the times.