If they are coming through the registrations, your problem may not be with bbpress itself.
Try using these plugins:
https://www.ads-software.com/extend/plugins/stop-spammer-registrations-plugin/
https://www.ads-software.com/extend/plugins/block-spam-by-math-reloaded/
I’m using them (along with cloudflare.com) on a multisite network and get very little spam. Today, I opened up my bbpress to anonymous posting. I’ll be testing it for a week with my protections in place to see how much spam I get, but those plugins+cloudflare (cloudflare and stop-spammers both use project honeypot) have kept my spam problem under control.
Another thing to think about is to create a new registration.php (I’m on BuddyPress, not sure what BBpress has) and modify the default text that is displayed on the registration page itself. I’ve found that spammers are doing searches for default registration page text if you change it and let google crawl your reg page for a couple weeks to get it fully indexed, you should see a reduction in spam.
I did a full write on handling spam on here, while it is dedicated to multisite/BP, you might be able to get some ideas https://premium.wpmudev.org/forums/topic/how-i-stopped-splogs-and-spam-and-a-suggestion-for-the-wpmu-dev-folks