bbPress is fully integrated with Akismet. Akismet is a very effective plugin and service that will catch various types of spam and a vast majority of it. This service is especially helpful if you also have an existing blog and allowed user to comment on blog posts on your WordPress site, as Akismet will also take of that.
What Akismet does for your bbPress forums is scan any new topic or reply before it is posted in your forums to determine if a bbPress post is spam with it’s scanning service that runs hundreds of tests. If the service catches a topic that is spam, you will see it in the WordPress backend in the topics section under spam, and the same process for replies.
This will make moderation much easier so that you do not have deal with spam as much you would need to without it. No more spammers disrupting your community and no more slaving away post after post marking posts as spam yourself.
Usually Akismet is pre-installed with any install of WordPress so you should have it already, but if you are one of the unlucky ones go to Plugins > Add New then you should see Akismet as one of the featured plugin, then install from there.
Akismet does require an API key that you can obtain by registering to their website.
Akismet is FREE for personal use, but is not for commercial use like you are selling a product on your website. Akismet offers varied plans for the service if you need to use it for commercial use.