perhaps a better solution overall would be to implement some sort of restriction on submissions. if you submit from a single domain more than X number of times in the last Y submissions, you're disallowed from submitting from that domain for another Z submissions.
I'm actually a bit suspicious that the following may be happening:
1) Karma-hungry users submit all of his articles as soon as they're posted without checking whether or not they've been submitted already.
2) Since submitting a page that's already been submitted results in an upvote, his articles make it to the front page and RSS feed without the usual oversight of people reading things in the new submissions list.
In effect those hypothetical karma-hungry users amount to a voting ring, but one that does not need to communicate --- or even realize that they are in a voting-ring: Members just notice that sometimes --- when they are by chance the first to submit --- they get a huge karma boost. And when they are not the first, they do not lose anything from supporting the other members.
Potential solution: Decouple. I.e. do not make submitting an automatic upvote for existing stories. Just forward to the existing item and let people upvote (or not) manually.