Currently I have cast of 288 downvotes, and I feel that I should not be limited to openly expressing that I believe a question or answer is bad/wrong/not useful.
Knowing this:
##Ratio Cap
Ratio Cap
An upvote/downvote ratio would be a nice way of dealing with this. It allows the people that legitimately want to downvote people to continue to do so, but makes sure that they are also actively upvoting people as well.
The problem would be the frequency of ratio calculation. If the calculation is cached, there are a large number of ways it could be abused.
##Moving Cap
Moving Cap
Another possible solution might be a moving cap. Every week or so, take a percentage of the current posts (questions/answers), and that would be the cap.
All in all, I would favor against imposing any sort of limit besides the increase in the amount of rep both the voter and the receiver are deducted.