IPB supports Recaptcha2 by Google, meaning that in most cases your users don't even have to type in random letters. Google automatically determine if the user is human or otherwise, and also supports KeyCaptcha
"IPS Spam Defense service" It's built into the software that links to all IPB sites. That checks key data points against known spammers. And when it detect an new spam, it goes to the unwanted list.
If all that fail (can happen) and one get in as an spammer then "One Click CleanUp" simple as going to profile and click "Mark as Spammer". The user's account will be immediately banned, and all of their content will be removed. It's like they were never there.
There are an total so far 6446152 total spammers that got automatically blocked from IPS Spam Defense to date