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we can make either @SteemNuke bot or #SteemNuke group for that, but we need some motivation for people to start fighting spam on their own too.

Yes, I agree. Downvoting is not used widely. People lose out on curation rewards if they use their power that way (since they might otherwise use it to upvote something good), plus there is the danger of getting into a flagging war with someone. It's far too easy for reputations to be impacted. I think when communities come, the owners/moderators will have more ability to hide the junk content, so maybe that will help.

maybe giving the same reward as up-voting a good post, for don-voting a spam post would motivate more people to participate in cleaning.