Why don't you publish the top 3 or 5 folks who up vote and down vote the most, maybe as an additional section on the leaders page. It might effect behavior interestingly.
PS: As an after-thought, social pressure may be more adaptable than software.
The karma-neutral idea sounds like a good one, but then you remember that it's easy to create new accounts... Ultimately, you just have to depend on people to do good or leave.