For me it's a combo of:
1) Spambot Stopper - prevents the typical Spambots from registering - and also ad-free
https://vborg.vbsupport.ru/showthread.php?t=294633
2) Spam-O-Matic - inspects the rest of them against the Stop Forum Spam database
3) also changing from an Image Verification/Re-Captcha to the Q&A seemed to help
see the Admin Control Panel-> Settings -> Human Verification Manager
ofc - all bets are off if you were looking for an actual "Spam" mod