You could implement a deposit system. People submitting reports pay a deposit fee that gets refunded immediately after the report is confirmed not to be slop. That way, the only people who pay the fee are the ones who submit slop reports.
Did you read the link? That idea is covered there.
Oh, whoops. I only saw that they mentioned charging a fee but thought it seemed hostile—I figured, seems like there's a way to mitigate that, but I guess they realized that too.