I've no direct knowledge at all into this, but... for my own forums I found that most of the comment spam tools I was using presumed IPv4. This was especially true of "spam from same network" type logic that these tools were using.

For example only: Stop Forum Spam only recently began testing their work for IPv6: https://www.stopforumspam.com/forum/viewtopic.php?pid=44371

I re-wrote my comment spam solution (not specifically for the comment spam problem, was doing some other stuff) and have long since enabled IPv6, but it's worth bearing in mind that network is one thing, sometimes what holds someone back from flipping the switch are the tools we use.