User X is upvoting user y's low quality posts 10x a day with a large stake. User y is mining a ton of coins with no effort. If we, as a community, decide that this is "bad behaviour" then we can use a downvote? How would this work?

If you are intent on using downvotes to solve that problem (assuming the above proposal is ratified), someone would need to step up and present a subsequent proposal detailing clear guidelines for how downvotes can or should be used to counter that behavior. Certainly there could be some circle-vote metrics created to make that objective rather than subjective. There are a few of us already actively working on developing metrics like that -- to increase transparency across the board.

However, for the time being, you could simply ask the tribe 'owner' to mute the offending accounts. Whether we like it or not, Hive-Engine tokens and tribes are fully centralized and whomever controls the issuing account pretty much rules the kingdom tribe.