I remember when I had this problem.. you could turn up the flood session to like 60... so they have to wait one minute per post... And then get some good moderators that can delete the posts. Therefore the spammers would have to register new E-mail, Account verify, and then wait 1 minute per post. Then of course you could delete the post, and ban their account in that amount of time.
Downside: Your user's wont post as much because of flood session, but if your board is active then get some moderators.
Usually the basic SPAM mods will get rid of the bots, and some more. But in my memory that doesn't do everything.