I assume that you already have an anti-spam plugin installed. Make sure that you're targeting the right terms so the anti-spam filter puts those comments into moderation and people can't see them. A good configuration and good terms may reduce the amount quite a bit.
Still, plenty of the spam here is handle by us, the moderators.
It may be worth to check Ips. Although banning an IP would mean that a possible real user may not be able to access your forum, if you are receiving plenty of spam from the same IP, it may be worth to consider a IP ban.