Absolutely nothing, which is why I over and over again come out 100% against this idea. It's silly when you can create multiple accounts, delegate, or vote trade. It might stop the laziest or most technically dis-inclined abusers, which just shifts the "booty" to the more technically savvy, determined abusers. It seems worse than doing nothing. The big abusers don't self-vote, they vote trade in cartels like BookingTeam.com:
The only solution I see is removing flagging from using the same power as upvoting. There is too much financial disincentive not to flag things, and it's already bad enough there is a lot of social and retaliation disincentive as well.
Right you are.
They even talk about this in the whitepaper. My post inspired by this thread might be of interest to you.
https://steemit.com/steem/@kyle.anderson/subjective-proof-of-work-some-rational-comments-on-the-self-voting-trend
Thanks, I'll check it out.