> A karma system
Upvoting and downvoting can and have been gamed by users and groups of users. It's a fine distraction to drive engagement in a social media space, but you cannot delegate moderation to the crowd without it turning into hot mess.
> and/or paid accounts.
At some point this might have been a good idea, but it doesn't stop bad actors from joining or returning through credit card fraud.
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Have you looked at tildes? It's a reddit/HN-style site, but with stronger moderation and no downvote button. But the most interesting feature is that you have to be invited to the site by somebody, and the chain of invites is tracked, so if there are a bunch of troublemakers that all seem to have a root at one individual, the moderators can get rid of the whole tree if need-be.