Markus Echterhoff

If you have a multisite setup, you normally cannot have the same bbPress forums across all sites of the network. Now you can.
Install this plugin and activate it on the main site and for each subsite that should use the main site’s forums, or network-activate to enable on all sub sites.
If you want both main site and sub site forums displayed on the sub site, set a different root slug and topics slug in the forum settings. After that, both slugs will be known on the sub site and display their respective forums.
BUT! There is a reason why you cannot normally do this and that is because the code base is not really supportive of it. I have monkey patched my way to the goal and I’m doing some evil black magic like hooking ‘query’ and running regular expressions on the query. All of this is only ever done to your forum pages, so be cool, give it a try. If it works for you: great. If it doesn’t: deactivate, relax, look for a different solution (or patch mine).
Profile Note: Profiles are not merged! Favorites, topics started etc show main/sub site only. If you want your users to see all their subscriptions in one place, you’ll have to write some custom code and likely patch my code too.
BuddyPress Note: It does work with network enabled BuddyPress, but they don’t play together nicely because of the BuddyPress/bbPress integration stuff. I have monkey patched this to work. If you do not have BuddyPress installed in /members/ you’ll have to monkey patch my monkey patch, please see the code. And as for the profiles: Same thing as with bbPress profiles.

Multisite bbPress Slave

Make sub sites use the main site's bbPress forums

Hitesh Chandwani

bbPress Google reCAPTCHA v2 is a free plugin/add-on for bbPress, that allow you to easily add Google reCAPTCHA v2 on your bbPress forum.
bbPress Google reCAPTCHA v2 prevent spam from creating new topics or replies. This plugin is built to integrate seamlessly with your bbPress enabled WordPress website.
bbPress Google reCAPTCHA v2 is very easy to use and work for Anonymous posting as well as Logged-in users.

reCAPTCHA for bbPress

I was getting way too much spam on bbPress forms until I added this plugin. Thank you, good sir!

Does exactly what I need

Google reCAPTCHA v2 for bbPress is a free plugin/add-on for bbPress, that allow you to easily add Google reCAPTCHA v2 on your bbPress forum.

Multisite bbPress Slave vs reCAPTCHA for bbPress