We should make a captcha that mines crypto in the browser. For example mine 1 valid share to a mining pool to my address and your post goes through.
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Though I'm not sure if there's still a mining algorithm around that works in the browser. I remember mining on Coinimp a couple years ago, but I'm not sure about its profitability.
However you could argue, that if someone spams your board, at least you're indirectly getting paid to moderate it.