BuddyPress.org

There is nothing protecting spammers from registering on testbp.org. I get some occasionally, but I nuke them with “mark as spam” as soon as they pop up. I found since with the new default theme spam has reduced significantly.

SI Captcha is a good spam prevention method. There really are a lot of options, but don’t search for “BuddyPress spam prevention”… this is not a BP issue, it’s a WordPress issue. You need to search for WordPress spam prevention instead.