1) enable email confirmation, it will stop robots joining your site and make numerous of post just after joining.
2) Disallow joining/login for some countries - http://www.boonex.com/forums/topic/Configure-DNS-Blocklists.htm
3) enable Akismet
4) limit number of blog posts per hour/day, for example don't allow to post more than one blog post per one hour, you can do it from admin membership levels
5) https://www.boonex.com/forums/topic/spam-spam-spam-spam-IDEA-.htm#157136