If you are getting hit with bots, I suggest the following:
This will stop most of the spam bots that auto-register:
https://vborg.vbsupport.ru/showthread.php?t=294633
This gets alot of human spammers because they still use early IE:
https://vborg.vbsupport.ru/showthread.php?t=268208
This stops registration by any device on blacklisted host names.
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.