How about minimum of 5 or 10 minutes between posts, which would help to exclude post bursters or bots having fake conversations with each other?

I would also exclude posts made on Off Topic and Archival. Maybe also exclude the marketplace boards? There are an awful lot of newbie accounts which only exist to post likely scams on the marketplace, and there are also a lot of sockpuppets created to give fake vouches to these scams.

Also exclude banned users as best you can.