Well the registration page has both Captcha and a verification question which will stump the majority of bots but some are partly human driven so they automatically fill in most of the data but also have a real person to answer the bits they need to.
I understand there are some plug ins for forum software which block black listed IP's but I don't know if any are available on MyBB (the current software). The last line of defense are our mods who tend to mop up anything that gets through pretty rapidly.