Simple, make it so that the higher the rewards are on a post, the more Vests would be required to increase a reward, the lower the reward, the more vests required to lower the reward.
I can write out the formula used to calculate an upvote here and show a potential of what I mean through graphs but seriously, it would work. The amount of MVests required to increase a vote by a specific value (say 1 SBD) should also increase exponentially creating an effective upper limit on posts. This is for witnesses to decide on how to implemement if they do but I know a couple of formulas that work
A consequence of this should also provide that posts that are upvoted to high amounts should not require as much MVests to decrease the value. I am not saying someone with 25 SP should be able to flag a $400 post for a dollor, I mean really their flag should count for 0.006 SBD still (or whatever it is) instead what I am saying is that it should take exponential MVests to upvote a reward high and that if we keep the current flagging scheme then by consequence it won't require as much SP to fight it. This will make it easier to fight abusive behavior with voting bots, self voting, and other things.
If this isn't clear enough I can make another post or more preferably have a discussion.
Tag a witness, repost. Whatever, lets solve the issue of people being able to abuse the system, unless the witnesses have no urge to solve that issue?