I've come across the CleanTalk anti-spam service
1) extremely effective on my blog
that was also inundated with spam comments and spam registrations,
2) very affordable: only $8 per year
I'd like to use the service for this forum as well. The plugin is now installed but it's not activated yet because I haven't bought a license for it yet (and there's no trial). I hope it will work as well on this forum as it does on my blog. That would mean we could finally allow new user registrations again.
Too bad the community seems to be dwindling... I wish I'd seen this plugin many years ago!