MyBB Community Forums

In the User Registration and Profile Options of the board settings, you can change the registration method to "Administrator Activation" to manually activate each new member.

In the end though, there will always be bots/spammers on any forum no matter how much anti bot tools you use, it's a never ending battle for forum owners.

for bots registration prevention methods , see responses here

I have 2 options for this, use google recaptcha and add the security question plugin.

Google recaptcha = http://www.google.com/recaptcha (you have to login with your google account and then make public and private keys for your URL to add them into settings=>general configuration while choosing the recaptcha option)
Security question plugin = http://mods.mybb.com/view/registration-s...y-question (you can add your own questions with multiple answers, use "|" to separate them, ex: two|2.