This is something I coded yesterday mainly see how integrating a script like miniBB with Akismet would happen. :-)
This plugin will reduce the amount of spam users that register to your miniBB forum. It can delete spam accounts as they are created and attempt to block those IP addresses as well.
To use this plugin you need an API key from https://akismet.com/
1) Download this file
2) Upload it to your miniBB's root directory
3) Add at the end of bb_plugins.php this code: require_once('bb_kuo_akismet.php');
4) Save Akismet key inside bb_kuo_akismet.php
5) Change also some other settings inside that file to your liking