Earlier this year there was a lot of talk of pay to DM, and I don't remember it being unviable. Pay to reply is the obvious antispam. User should be able to set price, ideally per post so can be varied.

SM has to be about defaults that deal with stuff. People HATE running around having to block/mute. They want systems that deal with it - pricing is the obvious one. Idiots are at least generally sensible when it comes to not wasting their money.