This counts as reducing human environmental impact (on Earth, at least), but I was asking specifically about ideas for reducing human population levels by a lot.

The answer is we don't, at least willingly. We should be optimistic rather than pessimistic.

The other answer is to get rich, which lowers birthrates faster than anything else. Beats a modest proposal.

Just to be clear, I agree with being optimistic rather than pessimistic. I was just wondering if the poster I responded to would be willing to be explicit about how he thinks the reduction of population he claims is needed would be accomplished.