Just limit the amount of funds that can be extracted from rewards. Sound investments are invariably based on increasing the value of the investment vehicle, and rewarded by capital gains. Investors have been so encouraged since prehistory, and this is the basic mechanism which has created our extant markets.

That will end bidbots, self-voting, and such without doubt. Ending extracting rewards by manipulating curation will allow curation to actually be based on content quality as judged by individuals, rather than parasitized by profiteers.

Anything else will continue the downward spiral.