You could start off by installing vbstopforumspam:
https://vborg.vbsupport.ru/showthrea...=stopforumspam
That will check a user's ip address, email address and username against the database at stopforumspam.com.
Then go through and find your users that you think are spammers.. delete the pms they sent then delete/ban the user.
The vbstopforumspam mod should stop most of them from joining in the future. I usually have it set up to ignore the username but to check ip and email and it works out great for me and others i've installed it for.
Rob