MyBB has been around for a while, and offers free and open source forum software. MyBB goes up against some stiff competition such as SMF or Simple Machines Forum. MyBB is arguably the easiest to use and modify forum, but it's also the absolute worst in preventing forum spam.
There have been many reports from MyBB users that their forums are under attack by spammers specifically targeting MyBB forums because they know the forums are easy to exploit and overwhelm. MyBB out of the box has almost no forum spam prevention, not even CAPTCHA or random questions during registration.
I took the time to setup a MyBB forum, and have been running it for over 3 years. During that time I have gone through all of the MyBB updates (when you log into the admin, there is a handy little tool to let you know if you are running the latest version, and if not, what the latest version is). All of the updates were to fix some kind of exploit, but none of them were for preventing forum spam.
I have been noticing in my analytics that spammers are searching for "Powered by MyBB" when targeting my forum. They are specifically looking for a MyBB forum because they know their automated activities will be able to overwhelm most board owners and moderators.
If MyBB wants to be successful, they need to release MyBB 1.6 with at least the two basic forms of trying to catch spammers at the registration level: Captcha, and asking random questions. Right now, the only MyBB spam prevention is to send a random password to the user to check their email (which doesn't work), send a confirmation email (which also doesn't work), or just let them instantly be activated.
Also, the admin only lets you see 20 users at a time, so trying to do a mass ban is very time consuming.
If you are looking for a forum that helps you in the quest of having a great community with minimal spam, stay away from the MyBB 1.4 series.