I like the heart of your idea, but as can be seen with pretty much every other social platform, the pieces just get moved around.
The Fake Votes Will Just Move Elsewhere
If you limit self-votes, then people will move onto creating sock puppet accounts, as you've suggested in your post.
If you limit the number of times people can vote within a time-window for a certain author, then they'll just create more sock puppet accounts and alternate them.
If you figure out someway to curtail that behavior, then they'll just pay other members to upvote them, or create upvote sharing communities (which exist for pretty much every social platform).
What Other Social Platforms Do
This is exactly why most social platforms often require phone, photo and ID verification. There are even services that exist where they provide a phone number for you to use for the verification of a fake account! They're attempting to stop these acts, but it's a game of cat and mouse.
Why I Think Steemit is Already Winning the War
Granted, I'm very new here, but this is my perspective on this: I think Steemit is getting it right in that they're trying to incentivize the creation of quality content, rather than spending all of their efforts on trying to shut down people who put lots of resources into gaming the system.
At the end of the day, whales aren't going to vote on their crappy posts. They can game until they're blue in the face, but they won't walk away with any meaningful amount of money. After a while of trying, I'm sure many will just give up.
Why are Spammers Here if Steemit's System is So Good?
Part of the problem is that a lot of the scammers seem to treat Steemit like a lot of the other social networks, where the votes are almost all equally valued. On Facebook, my like is just as valuable as any other person's like.
That's what makes gaming Facebook with a sock puppet account so powerful. Steemit's system has eliminated that problem, and I don't think it will take spammers long to figure that out.
Good points. I guess the harder it becomes to do and the less incentivized it becomes, the less of an issue abuse becomes...