Do MyBB forums have a queued posts feature like phpBB boards have?
Queued makes it virtually impossible for people to spam. A staff-member needs to manually confirm at least one post by a new user before the member's posts go live. If a new member signs up and posts spam, it doesn't matter much because nobody on the board will see the spam but staff. You can simply disapprove post(s) by spammers and ban them without anyone on the board having to see anything.
The queued posts feature on phpBB boards is essential in my opinion; hardly anyone uses it -- it's a shame.