2
Unfortunately, there will be always spammers who will try to register
See this thread:
https://xoops.org/modules/newbb/viewtopic.php?post_id=353391#forumpost3533911) set up the Profile correctly, with saving after 2nd step
2) set up the text captcha with text option, so it ask a math question
3) activate Protector for the spam on Forums, and set it "ban" there (important)
4) from time to time change the mandatory fields
5) update to 2.5.7 RC from SVN
That's all what we do here, and we have very few spammers actually.
But the focus is actually to make sure that they don't post spamming messages on the Forums. If few of them register, I am not losing sleep over that
Most of them register, but never connect, anyway....