CMS
In the past I have operated various CMS services.
PHP-Fusion
One of my favorites is a system called PHP-Fusion.
I operated PHP-Fusion over a two year period from my home server. No spammer logins were created. PHP-Fusion seemed to have the ability of remaining amazingly spam free.
PHPBBS
I have also operated phpBBS a few times.
Unfortunately, I haven’t had very good results with phpBBS. The spammers somehow manage to get past the security eventually. Next thing you know your forums are full of spam.
bigslugbox
I’m currently giving phpBBS another go at http://bigslugbox.com
The latest versions of the various CMS’s around the place provide advanced levels of protection against spammers.
Make sure you always install the latest version of any software.
Previous versions may/will have security holes that will compromise your ability to remain spam free.