I've gutted out all the stuff I'd tried adding to stop spam as universally ineffective, and have replaced it with CleanTalk's service.
If you have issues registering or accessing the forums after this change, please use the contact form here:https://cutcodedown.com/contact
... and include your username, and if this is a registration issue the e-mail address you tried to register from.
A lot of people "talked up" how good ClearTalk is. I'm going to give it the week of free trial to see if it's worth buying. Eight bucks a year is piss in a barrel if it works.
I was very pleasantly surprised how well the package for SMF went in with no conflicts.