Instead of giving free downvotes, why not figure out a way to punish the behaviour that should be downvoted?
For example, I have been downvoting bots which tag me in curation spam posts, just a small downvote and the micro-cents it costs doesn't bother me as much as the spam. A system should be designed to prevent minuscule votes for curation as well.
You could read my post about Spam for ideas.
Serial commenters and upvoters should also be dealt with. An account giving micro upvotes to only large Steem holders hundreds of times a day does nothing for the blockchain. Low-quality posts which use bid bots are already policed, why not just adjust curation to the point where these bots are put out of business.