If we are identifying a voting pattern we don't like, we should be stopping it entirely.
All downvotes should be consistent. They are all worth equal amounts and they cause identical amounts of rep-burn to the user. If you start costing people more to downvote once they've been identified as serial downvoters, then how do you recognize a 'good' downvote from them?
There was once a user that answered a question of mine very well, and I upvoted him, and I was interested to read some of the other things he had written. I read a few of them, and there were some very blatant impolite suggestions, and some incorrect answers. After reading a few of his answers, I'd given 4 downvotes and 2 upvotes. That user was "SO Sucks". He's since changed his name.
But, the point is that I wasn't being malicious, I had no grudge against this user, I just found answers that I thought were worthy of downvotes. Overall, after doing that voting, I ended up down 4 rep points, and he was up 12.
If your suggestion is put in place, I would be down something like 10 points, and he'd still be up 12.
Based on the comments, I think a good solution would be to have a cap on the number of votes you can give to any one user in a particular day.
With the red-pop-up we all recognize as the "I can't let you do that" box. That way, the votes remain consistent in their value and their and effect.
Obviously, we'd want to have a flag if they were mostly negative as well, because most users have upvote behaviour that FAR outweighs their number of downvotes.
Unfortunately, this would make it difficult to downvote spammers into oblivion, becuase if they post 6 times, we can't do anything about them.
(Scratch that, we can just flag their butts into oblivion)