Running a forum can become tedious if your time is sucked up deleting thread after thread of spam. I’ve found the following combination of plugins invaluable and wanted to share them with you.
Akismet
Chances are, if you run a blog, you’ve already heard of this website. The plugin checks all new posts against Akismet’s database and if it detects a post it deems as spam, it’s placed in a queue for you to approve or deny.
Plugin Url: http://mods.mybb.com/view/akismet-(spam-protection) Still works with 1.6.* – Changing the version number in text editor is required.
StopForumSpam
StopForumSpam is a very fitting name for this plugin, as for me, it’s stopped an insane amount of spammers signing up. The plugin works by checking a new signup’s IP address, username and email against their spam database.
To give you a small idea of the effectiveness of this plugin; here’s a screenshot of my logfile:

StopForumSpam Log
Plugin URL: http://mods.mybb.com/view/stopforumspam-com-for-mybb
Registration Security Question
This plugin allows you to set your own security questions to ask on signup. This is a great way to combat bots as the questions can be literally anything you choose.
Plugin URL: http://mods.mybb.com/view/registration-security-question
Other steps to take:
Bots may try to stay under the radar by PMing members instead of posting. I combat this by making new members unable to send any private messages. Their accounts are automatically upgraded and provisioned at 15 posts, by which time I hope to be able to tell their intentions.