I don't think you can change user behavior like this.

You can give them a "venting sink" though. Instead of having a downvote button that just downvotes, have it pop up a little menu asking for a downvote reason, with "spam" and "disagree" as options. You could then weigh downvotes by which option was selected, along with an algorithm to discover "user honesty" based on whether their downvotes correlate with others or just with the people on their end of the political spectrum, a la Birdwatch.