This will stop most of the spam bots that auto-register:
https://vborg.vbsupport.ru/showthread.php?t=294633
This stops registration by any device on blacklisted host names: (You compile the list. I have a good one I will give you via PM. This is effective against both human spammers and auto bot spammers)
https://vborg.vbsupport.ru/showthread.php?t=279110
NONE of the above use captchas, puzzles, questions or other such nonsense that merely annoys legitimate users, and have been rendered obsolete anyway for the most part, by autospam programs such as XRumer.
The era of Big Spam is over.