Well there's many ways to go about this... You can check the built-in anti-spam options in vB4 and/or go further by installing a mod in particular one of these:
https://vborg.vbsupport.ru/showthread.php?t=230921
https://vborg.vbsupport.ru/showthread.php?t=239307
https://vborg.vbsupport.ru/showthrea...ight=stop+spam
There's also useful threads on here and vbulletin.com you should search for keywords related to spam, spam management, and simply "stop spam" on both sites and Google with "vBulletin 4" in the same search. As I said there's many ways to try and prevent spam, none are 100% imo.