Serial-downvoting is already detected and prevented - only without the popup, something that has been discussed on the StackOverflow podcast a few times (I cannot find mention of it in the transcript however)
If I recall, the reasoning is basically if you tell the user "we've stopped counting your votes again $user", they'll work a way around it. If you silently ignore them they continue on, not causing any real damage..
Atwood: [...] There was a gem of something in there that's a little bit interesting that we've thought about which is, we have had incidents of people who--they get pissed off at another user so they go in and downvote everything that user has ever done. Right? First of all, that's not as bad as it sounds because an upvote is worth five times as much as a downvote, so it would take five downvotes to cancel out one upvote so there's a limited amount of damage you can do. Plus you only have 30 votes in a 24 hour period anyway, so you can only do that on 30 posts by that user. But still, it is aggravating and I totally empathize with people that are complaining because this is annoying. Somebody told me, it might have been you--I don't remember, Mr. Reddit fan, but on Reddit they track this and if they see a user consistently downvoting another user, they basically silently undo all those votes, like those don't even count.
from Podcast 026
There was a better discussion of this subject, but I cannot find it (the above quote took about 20 searches to find..)