Policing the system with downvotes means punishing the minnows who bought the upvotes from another minnow who pays for delegation off a whale.
So downvoting these minnows doesn't stop that whale getting the money. In fact it just screams scam. Because these are ordinary social media users spending their money on self promotion and they won't get their money back when they get punished for purchasing the service.
Charging for upvotes and delegation is actually what's encouraging users to use the platform this way because they don't have the incentive to care about the platform.
Minnows don't have the same incentives as whales and dolphins to make the platform attractive to investors. So you can't change a minnows mind about spending the money they spent on upvotes or delegation on their own shitposts. If that's the best thing whales have to offer that's what they're going to go for and nobody can blame them for that.
So here's my solution for all the free market lovers. Simple solution for those with the power is to offer cheaper rates. Of course minnows spending $50 a week are going to try to make that money back and the current rates don't even allow that. To compete with whales offering delegations EMPLOY SOME MINNOWS who are going to use their delegation responsibly. Use steemreports to check that they're voting according to your standards.
As for people selling upvotes I don't agree with it benefiting your investments but it is a free market so I accept that. What I would say could be better would be to pay for attention rather than automated upvotes and I would hope maybe steemit inc could help with this by disabling notifications of transfers in the wallet so that features can be developed enabling users to put a price on their attention.
As for the whales policing the system the tools and incentives are there already. If you're invested in STEEM you don't need any added incentive to "police" the system. Any policing of the system is for the purpose of making the STEEM network more attractive to potential investors and users. A whale with 500'000 SP gains $5'000 every time the price of STEEM goes up by 1c.