Support Forums

Hmm there are few more extra options for you to prevent spam bot attack.

1.generic captcha provided by mybb is really behind the bars.so u need a recaptcha plugin for that.
Here is it :
http://mods.mybb.com/view/recaptcha-ii

2.stop forum spam is really good,install it,but a problem is it recognises normal users as spammers if they make quick posts.
Here is another plugin for u :
Goodbye spammer :
http://mods.mybb.com/view/goodbye-spammer

This helps u delete all spam in a button click

Also make block the email domains that has short expiry like 30 minutes email.
this will help u to get ur forum protected from spammers

I have never gotten my forums spammed Must really suck lololoolololo

Have you tried Akismet? It works well on WordPress and SMF at least. Should work as well on MyBB too

Bots find your site using the same method as scrapebox or even using scrape box.

"Powered By MyBB, © 2002-2011 MyBB Group."

Change this too an image so they find it harder to scrape your site if you are having problems. Mybb said it is fine as long as you have it there.