Our friends at Tricycle will push through a zero-based karma system (no self-voting possible) sometime this evening. At present this will only cover future posts/comments - they may go back and revise history some time in the indefinite future (or not), but apparently that would be overly complicated for now. We'll see how this works.
We've just deployed another related change: Now you can only cast as many down votes as your karma value.
Then it should be possible to see how many down votes I've cast, to know how many are left. In practice, I expect that the number of downvotes is significantly less than the amount of Karma, but I'm not sure. If this is true, then the limitation is roughly equivalent to simply prohibiting people with Karma less than a threshold to downvote.