If your core files are compromised then it's possible to register without going through the correct channel, if thats not the case then its possible your Q&A answers are already cached with spammers/bots and thats how they are getting by. So, you need to add extra measures to stop the automated registrations - do you have email confirmation turned on?
Seriously, search here and install some stuff like the following:
https://vborg.vbsupport.ru/showthread.php?t=176481
https://vborg.vbsupport.ru/showthread.php?t=261492
https://vborg.vbsupport.ru/showthread.php?t=264932
https://vborg.vbsupport.ru/showthread.php?t=201466
There are loads, make use of them