EMail verificaiton is a nessessity in my opinion. Also, enabling the CAPTCHA which comes with 7.1 is as well.
Most of the posts are likely due to cross site posting as well, which is what the referrer check is designed to thwart.
When a user is banned they CAN sign up again, under a differant username with a differant password; you'll need to ban their IP or their IP Segment if they become a re-occouring issue.
post moderation queue is a valid option, should you actually have the staff to monitor it all the time on a busy forum.
