Sorry for such a late reply @steemchiller

The problem I see with a blacklist is that it comes with much maintenance work

My impression is, that it's easier to maintain blacklist than to seek and downvote abusers on daily basis.

I've few good friends, who are right now "milking steem" like crazy. And when I asked them if they aren't worried to be downvoted - I received interesting answer: flaggers will get bored sooner or later.

That's the problem. Those who abuse system, they go "quiet" for several days after been downvoted and then they are coming back. Perhaps unnoticed. Or they simply carry on, knowing that they will win at the end of the day.
@ haejin is great example.

The other problem is that you need 10times more downvoting power (1 free downvote daily) to balance voting power. Meaning: if someone has 100k SP and is self-upvoting 10 of his spam-posts, then you need 10 people with 100k SP to downvote it to zero. That's a battle which cannot be won.

A similar and much easier to implement idea would be some kind of maximum payout amount per week for all accounts.

Interesting idea. Worth considering.

Yours, Piotr